Руководство по PHP
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eval

(PHP 3, PHP 4 )

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. In 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.

Пример 1. eval() example - simple text merge 

<?php
$string = 'cup';
$name = 'coffee';
$str = 'This is a $string with my $name in it.<br>';
echo $str;
eval ("\$str = \"$str\";");
echo $str;
?>

The above example will show: 
This is a $string with my $name in it.
This is a cup with my coffee in it.

Подсказка: Как и с любой другой функцией, осуществляющей вывод непосредственно в броузер, вы можете использовать функции контроля вывода, чтобы перехватывать выводимые этой функцией данные и сохранять их, например, в строковой переменной.

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