(PHP 4, PHP 5)
eval — Evaluate a string as PHP code
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.
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 .
eval() returns NULL unless return is called in the evaluated code, in which case the value passed to return is returned. If there is a parse error in the evaluated code, eval() returns FALSE and execution of the following code continues normally. It is not possible to catch a parse error in eval() using set_error_handler().
Example #1 eval() example - simple text merge 以上例程会输出：
$string = 'cup';
$name = 'coffee';
$str = 'This is a $string with my $name in it.';
echo $str. " ";
eval("$str = "$str";");
echo $str. " ";
This is a $string with my $name in it.
This is a cup with my coffee in it.
Example #1 eval() example - simple text merge
Note: 因为是一个语言构造器而不是一个函数，不能被 可变函数 调用。
In case of a fatal error in the evaluated code, the whole script exits.