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

示範 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"; ?>`
The above example will show:
    This is a $string with my $name in it.
    This is a cup with my coffee in it.

> 心得:任何直接把輸出到瀏覽器的結果，可使用輸出控制函數來擷取此函數的輸出，再將之存入一個字串(舉例說明)。

See also call_user_func().

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