PHP Manual
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# eval

(PHP 3, PHP 4, PHP 5 )

eval -- Evaluate a string as PHP code

## Description

mixed eval ( string code_str )

eval() evaluates the string given in code_str as PHP code. Among other things, this can be useful for storing code in a database text field for later execution.

There are some factors to keep in mind when using eval(). Remember that the string passed must be valid PHP code, including things like terminating statements with a semicolon so the parser doesn't die on the line after the eval(), and properly escaping things in code_str.

Also remember that variables given values under eval() will retain these values in the main script afterwards.

A return statement will terminate the evaluation of the string immediately. As of PHP 4, eval() returns `NULL` unless return is called in the evaluated code, in which case the value passed to return is returned. In case of a parse error in the evaluated code, eval() returns `FALSE`. In case of a fatal error in the evaluated code, the whole script exits. In PHP 3, eval() does not return a value.

例子 1. eval() example - simple text merge
`<?php $string = 'cup'; $name = 'coffee'; $str = 'This is a $string with my $name in it.'; echo $str. "\n"; eval("\$str = \"$str\";"); echo $str. "\n"; ?>`
上例将输出：
    This is a $string with my $name in it.
    This is a cup with my coffee in it.

> 提示: 为了防止程序直接将结果输出到浏览器，可以使用输出控制函数（output-control functions）来捕获此函数的输出，并把它们保存到一个例如 string 类型的变量中。

See also call_user_func().

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