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<chapter xml:id="language.expressions" xmlns="http://docbook.org/ns/docbook">
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<title>Expressions</title>
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<simpara>
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Expressions are the most important building blocks of PHP. In PHP,
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almost anything you write is an expression. The simplest yet
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Expressions are the most important building blocks of PHP. In PHP,
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almost anything you write is an expression. The simplest yet
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most accurate way to define an expression is "anything that has a
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value".
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</simpara>
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<simpara>
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The most basic forms of expressions are constants and variables.
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When you type "<varname>$a</varname> = 5", you're assigning '5' into
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<varname>$a</varname>. '5', obviously,
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has the value 5, or in other words '5' is an expression with the
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value of 5 (in this case, '5' is an integer constant).
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When you type <code>$a = 5</code>, you're assigning <code>5</code> into
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<varname>$a</varname>. <code>5</code>, obviously,
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has the value 5, or in other words <code>5</code> is an expression with the
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value of 5 (in this case, <code>5</code> is an integer constant).
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</simpara>
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<simpara>
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After this assignment, you'd expect <varname>$a</varname>'s value to be 5 as
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well, so if you wrote <varname>$b</varname> = <varname>$a</varname>, you'd expect it to behave just as
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if you wrote <varname>$b</varname> = 5. In other words, <varname>$a</varname> is an expression with the
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value of 5 as well. If everything works right, this is exactly
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well, so if you wrote <code>$b = $a</code>, you'd expect it to behave just as
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if you wrote <code>$b = 5</code>. In other words, <varname>$a</varname> is an expression with the
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value of 5 as well. If everything works right, this is exactly
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what will happen.
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</simpara>
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<para>
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Slightly more complex examples for expressions are functions. For
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Slightly more complex examples for expressions are functions. For
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instance, consider the following function:
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<informalexample>
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<programlisting role="php">
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Assuming you're familiar with the concept of functions (if you're
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not, take a look at the chapter about <link
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linkend="language.functions">functions</link>), you'd assume
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that typing <literal>$c = foo()</literal> is essentially just like
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writing <literal>$c = 5</literal>, and you're right. Functions
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are expressions with the value of their return value. Since <literal>foo()</literal>
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returns 5, the value of the expression '<literal>foo()</literal>' is 5. Usually
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that typing <code>$c = foo()</code> is essentially just like
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writing <code>$c = 5</code>, and you're right. Functions
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are expressions with the value of their return value. Since <code>foo()</code>
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returns 5, the value of the expression '<code>foo()</code>' is 5. Usually
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functions don't just return a static value but compute something.
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</simpara>
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<simpara>
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Of course, values in PHP don't have to be integers, and very often
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they aren't. PHP supports four scalar value types: <type>integer</type>
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they aren't. PHP supports four scalar value types: <type>int</type>
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values, floating point values (<type>float</type>), <type>string</type>
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values and <type>boolean</type> values (scalar values are values that you
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values and <type>bool</type> values (scalar values are values that you
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can't 'break' into smaller pieces, unlike arrays, for instance). PHP also
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supports two composite (non-scalar) types: arrays and objects. Each of
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these value types can be assigned into variables or returned from functions.
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</simpara>
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<simpara>
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PHP takes expressions much further, in the same way many other languages
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do. PHP is an expression-oriented language, in the
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sense that almost everything is an expression. Consider the
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example we've already dealt with, '<varname>$a</varname> = 5'. It's easy to see that
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do. PHP is an expression-oriented language, in the
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sense that almost everything is an expression. Consider the
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example we've already dealt with, <code>$a = 5</code>. It's easy to see that
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there are two values involved here, the value of the integer
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constant '5', and the value of <varname>$a</varname> which is being updated to 5 as
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well. But the truth is that there's one additional value involved
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here, and that's the value of the assignment itself. The
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constant <code>5</code>, and the value of <varname>$a</varname> which is being updated to 5 as
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well. But the truth is that there's one additional value involved
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here, and that's the value of the assignment itself. The
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assignment itself evaluates to the assigned value, in this case 5.
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In practice, it means that '<varname>$a</varname> = 5', regardless of what it does,
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is an expression with the value 5. Thus, writing something like
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'<varname>$b</varname> = (<varname>$a</varname> = 5)' is like writing
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'<varname>$a</varname> = 5; <varname>$b</varname> = 5;' (a semicolon
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marks the end of a statement). Since assignments are parsed in a
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right to left order, you can also write '<varname>$b</varname> = <varname>$a</varname> = 5'.
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In practice, it means that <code>$a = 5</code>, regardless of what it does,
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is an expression with the value 5. Thus, writing something like
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<code>$b = ($a = 5)</code> is like writing
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<code>$a = 5; $b = 5;</code> (a semicolon
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marks the end of a statement). Since assignments are parsed in a
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right to left order, you can also write <code>$b = $a = 5</code>.
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</simpara>
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<simpara>
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Another good example of expression orientation is pre- and
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post-increment and decrement. Users of PHP and many other
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languages may be familiar with the notation of <literal>variable++</literal> and
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<literal>variable--</literal>. These are <link linkend="language.operators.increment">
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increment and decrement operators</link>. In PHP, like in C, there
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post-increment and decrement. Users of PHP and many other
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languages may be familiar with the notation of <code>variable++</code> and
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<code>variable--</code>. These are <link linkend="language.operators.increment">
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increment and decrement operators</link>. In PHP, like in C, there
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are two types of increment - pre-increment and post-increment.
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Both pre-increment and post-increment essentially increment the
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variable, and the effect on the variable is identical. The
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variable, and the effect on the variable is identical. The
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difference is with the value of the increment expression.
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Pre-increment, which is written '++<varname>$variable</varname>', evaluates to the
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Pre-increment, which is written <code>++$variable</code>, evaluates to the
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incremented value (PHP increments the variable before reading its
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value, thus the name 'pre-increment'). Post-increment, which is
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written '<varname>$variable</varname>++' evaluates to the original value of
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$variable, before it was incremented (PHP increments the variable
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value, thus the name 'pre-increment'). Post-increment, which is
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written <code>$variable++</code> evaluates to the original value of
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<varname>$variable</varname>, before it was incremented (PHP increments the variable
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after reading its value, thus the name 'post-increment').
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</simpara>
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<simpara>
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The language also supports a set of strict equivalence operators: ===
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(equal to and same type) and !== (not equal to or not same type).
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These expressions are most commonly used inside conditional execution,
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such as <literal>if</literal> statements.
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such as <code>if</code> statements.
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</simpara>
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<simpara>
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The last example of expressions we'll deal with here is combined
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operator-assignment expressions. You already know that if you
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operator-assignment expressions. You already know that if you
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want to increment <varname>$a</varname> by 1, you can simply write
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'<varname>$a</varname>++' or '++<varname>$a</varname>'.
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<code>$a++</code> or <code>++$a</code>.
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But what if you want to add more than one to it, for instance 3?
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You could write '<varname>$a</varname>++' multiple times, but this
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is obviously not a very efficient or comfortable way. A much more
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common practice is to write '<varname>$a</varname> =
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<varname>$a</varname> + 3'. '<varname>$a</varname> + 3' evaluates
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You could write <code>$a++</code> multiple times, but this
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is obviously not a very efficient or comfortable way. A much more
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common practice is to write <code>$a =
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$a + 3</code>. <code>$a + 3</code> evaluates
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to the value of <varname>$a</varname> plus 3, and is assigned back
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into <varname>$a</varname>, which results in incrementing <varname>$a</varname>
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by 3. In PHP, as in several other languages like C, you can write this
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by 3. In PHP, as in several other languages like C, you can write this
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in a shorter way, which with time would become clearer and quicker to
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understand as well. Adding 3 to the current value of <varname>$a</varname>
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can be written '<varname>$a</varname> += 3'. This means exactly
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can be written <code>$a += 3</code>. This means exactly
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"take the value of <varname>$a</varname>, add 3 to it, and assign it
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back into <varname>$a</varname>". In addition to being shorter and
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clearer, this also results in faster execution. The value of
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'<varname>$a</varname> += 3', like the value of a regular assignment, is
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clearer, this also results in faster execution. The value of
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<code>$a += 3</code>, like the value of a regular assignment, is
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the assigned value. Notice that it is NOT 3, but the combined value
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of <varname>$a</varname> plus 3 (this is the value that's
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assigned into <varname>$a</varname>). Any two-place operator can be used
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in this operator-assignment mode, for example '<varname>$a</varname> -= 5'
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(subtract 5 from the value of <varname>$a</varname>), '<varname>$b</varname> *= 7'
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assigned into <varname>$a</varname>). Any two-place operator can be used
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in this operator-assignment mode, for example <code>$a -= 5</code>
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(subtract 5 from the value of <varname>$a</varname>), <code>$b *= 7</code>
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(multiply the value of <varname>$b</varname> by 7), etc.
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</simpara>
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<para>
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</para>
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<simpara>
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Some expressions can be considered as statements. In
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this case, a statement has the form of '<literal>expr ;</literal>' that is, an
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expression followed by a semicolon. In <literal>'$b = $a = 5;'</literal>,
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<literal>'$a = 5'</literal> is a valid expression, but it's not a statement
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by itself. <literal>'$b = $a = 5;'</literal> however is a valid statement.
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this case, a statement has the form of '<code>expr ;</code>' that is, an
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expression followed by a semicolon. In <code>$b = $a = 5;</code>,
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<code>$a = 5</code> is a valid expression, but it's not a statement
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by itself. <code>$b = $a = 5;</code>, however, is a valid statement.
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</simpara>
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<simpara>
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One last thing worth mentioning is the truth value of expressions.
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