Some popular tags and entities

<filename>          filenames
<constant>          constants
<varname>           variables
<parameter>         a function's parameter/argument
<function>          functions, this links to function pages or bolds if
                    already on the function's page.  it also adds ().

<literal>           teletype/mono-space font <tt>
<emphasis>          italics
<example>           see HOWTO, includes many other tags.
<link>              internal manual links
                    <link linkend="language.variables">variables</link>

<link>              external links via global.ent
                    <link xlink:href="&spec.cookies;">mmm cookies</link>

<type>              types, this links to the given types manual
                    page: <type>object</type> -> php.net/types.object

&return.success;    see: language-snippets.ent
&true;              <constant>TRUE</constant>
&false;             <constant>FALSE</constant>
&php.ini;           <filename>php.ini</filename>

Be sure to check out entities/global.ent (in the doc-base repo) and language-snippets.ent (located within each language's repo) for more information for entities and URLs.