php-doc-en/reference/misc/functions/eval.xml

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<refentry xml:id="function.eval" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>eval</refname>
  <refpurpose>Evaluate a string as PHP code</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>mixed</type><methodname>eval</methodname>
   <methodparam><type>string</type><parameter>code</parameter></methodparam>
  </methodsynopsis>
  <para>
   Evaluates the given <parameter>code</parameter> as PHP.
  </para>
  <caution>
   <para>
    The <function>eval</function> language construct is <emphasis>very dangerous</emphasis>
    because it allows execution of arbitrary PHP code. <emphasis>Its use thus is
    discouraged.</emphasis> If you have carefully verified that there is no other option
    than to use this construct, pay special attention <emphasis>not to pass any user
    provided data</emphasis> into it without properly validating it beforehand.
   </para>
  </caution>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>code</parameter></term>
     <listitem>
      <para>
       Valid PHP code to be evaluated.
      </para>
      <para>
       The code must not be wrapped in opening and closing
       <link linkend="language.basic-syntax.phpmode">PHP tags</link>, i.e.
       <literal>'echo "Hi!";'</literal> must be passed instead of
       <literal>'&lt;?php echo "Hi!"; ?&gt;'</literal>. It is still possible to leave and
       re-enter PHP mode though using the appropriate PHP tags, e.g.
       <literal>'echo "In PHP mode!"; ?&gt;In HTML mode!&lt;?php echo "Back in PHP mode!";'</literal>.
      </para>
      <para>
       Apart from that the passed code must be valid PHP. This includes that all statements
       must be properly terminated using a semicolon.
       <literal>'echo "Hi!"'</literal> for example will cause a parse error, whereas
       <literal>'echo "Hi!";'</literal> will work.
      </para>
      <para>
       A <literal>return</literal> statement will immediately terminate the
       evaluation of the code. 
      </para>
      <para>
       The code will be executed in the scope of the code calling <function>eval</function>. Thus any
       variables defined or changed in the <function>eval</function> call will remain visible after
       it terminates.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   <function>eval</function> returns &null; unless 
   <literal>return</literal> is called in the evaluated code, in which case
   the value passed to <literal>return</literal> is returned. As of PHP 7, if there is a
   parse error in the evaluated code, <function>eval</function> throws a ParseError exception.
   Before PHP 7, in this case <function>eval</function> returned
   &false; and execution of the following code continued normally. It is
   not possible to catch a parse error in <function>eval</function>
   using <function>set_error_handler</function>.
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>eval</function> example - simple text merge</title>
    <programlisting role="php">
<![CDATA[
<?php
$string = 'cup';
$name = 'coffee';
$str = 'This is a $string with my $name in it.';
echo $str. "\n";
eval("\$str = \"$str\";");
echo $str. "\n";
?>
]]>
    </programlisting>
    &example.outputs;
    <screen>
<![CDATA[
This is a $string with my $name in it.
This is a cup with my coffee in it.
]]>
    </screen>
   </example>
  </para>
 </refsect1>

 <refsect1 role="notes">
  &reftitle.notes;

  &note.language-construct;

  &tip.ob-capture;
  <note>
   <para>
    In case of a fatal error in the evaluated code, the whole script exits.
   </para>
  </note>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>call_user_func</function></member>
   </simplelist>
  </para>
 </refsect1>
</refentry>

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