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# eval

(PHP 3, PHP 4, PHP 5)

eval -- Evaluate a string as PHP code

## 说明

mixed eval ( string code_str )

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`. To mix HTML output and PHP code you can use a closing PHP tag to leave PHP mode.

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

## 参数

`code_str`

  The code string to be evaluated. `code_str` does not have to contain PHP Opening tags.

  A `return` statement will immediately terminate the evaluation of the string .

## 返回值

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 PHP 3, eval() does not return a value.

## 范例

例 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.

## 注释

> 注意: 由于这是一个语言结构而非函数，因此它无法被变量函数调用。

> 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。

> 注意: In case of a fatal error in the evaluated code, the whole script exits.

## 参见

call_user_func()

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