Kepp the following Quote in mind:
If eval() is the answer, you're almost certainly asking the
wrong question. -- Rasmus Lerdorf, BDFL of PHP
eval
(PHP 4, PHP 5, PHP 7, PHP 8)
eval — Evaluate a string as PHP code
Description
Evaluates the given code as PHP.
The code being evaluated inherits the variable scope of the line on which the eval() call occurs. Any variables available at that line will be available for reading and modification in the evaluated code. However, all functions and classes defined will be defined in the global namespace. In other words, the compiler considers the evaluated code as if it were a separate included file.
Caution
The eval() language construct is very dangerous because it allows execution of arbitrary PHP code. Its use thus is discouraged. If you have carefully verified that there is no other option than to use this construct, pay special attention not to pass any user provided data into it without properly validating it beforehand.
Parameters
-
code -
Valid PHP code to be evaluated.
The code must not be wrapped in opening and closing PHP tags, i.e.
'echo "Hi!";'must be passed instead of'<?php echo "Hi!"; ?>'. It is still possible to leave and re-enter PHP mode though using the appropriate PHP tags, e.g.'echo "In PHP mode!"; ?>In HTML mode!<?php echo "Back in PHP mode!";'.Apart from that the passed code must be valid PHP. This includes that all statements must be properly terminated using a semicolon.
'echo "Hi!"'for example will cause a parse error, whereas'echo "Hi!";'will work.A
returnstatement will immediately terminate the evaluation of the code.The code will be executed in the scope of the code calling eval(). Thus any variables defined or changed in the eval() call will remain visible after it terminates.
Return Values
eval() returns null unless
return is called in the evaluated code, in which case
the value passed to return is returned. As of PHP 7, if there is a
parse error in the evaluated code, eval() throws a ParseError exception.
Before PHP 7, in this case eval() returned
false and execution of the following code continued normally. It is
not possible to catch a parse error in eval()
using set_error_handler().
Examples
Example #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 output:
This is a $string with my $name in it. This is a cup with my coffee in it.
Notes
Note: Because this is a language construct and not a function, it cannot be called using variable functions, or named arguments.
Tip
As with anything that outputs its result directly to the browser, the output-control functions can be used to capture the output of this function, and save it in a string (for example).
Note:
In case of a fatal error in the evaluated code, the whole script exits.
add a note
User Contributed Notes 20 notes
Anonymous ¶
20 years ago
lord dot dracon at gmail dot com ¶
9 years ago
Inception with eval()
<pre>
Inception Start:
<?php
eval("echo 'Inception lvl 1...\n'; eval('echo \"Inception lvl 2...\n\"; eval(\"echo \'Inception lvl 3...\n\'; eval(\'echo \\\"Limbo!\\\";\');\");');");
?>
Jeremie LEGRAND ¶
7 years ago
At least in PHP 7.1+, eval() terminates the script if the evaluated code generate a fatal error. For example:
<?php
@eval('$content = (100 - );');
?>
(Even if it is in the man, I'm note sure it acted like this in 5.6, but whatever)
To catch it, I had to do:
<?php
try {
eval('$content = (100 - );');
} catch (Throwable $t) {
$content = null;
}
?>
This is the only way I found to catch the error and hide the fact there was one.
bohwaz ¶
12 years ago
If you want to allow math input and make sure that the input is proper mathematics and not some hacking code, you can try this:
<?php
$test = '2+3*pi';
// Remove whitespaces
$test = preg_replace('/\s+/', '', $test);
$number = '(?:\d+(?:[,.]\d+)?|pi|π)'; // What is a number
$functions = '(?:sinh?|cosh?|tanh?|abs|acosh?|asinh?|atanh?|exp|log10|deg2rad|rad2deg|sqrt|ceil|floor|round)'; // Allowed PHP functions
$operators = '[+\/*\^%-]'; // Allowed math operators
$regexp = '/^(('.$number.'|'.$functions.'\s*\((?1)+\)|\((?1)+\))(?:'.$operators.'(?2))?)+$/'; // Final regexp, heavily using recursive patterns
if (preg_match($regexp, $q))
{
$test = preg_replace('!pi|π!', 'pi()', $test); // Replace pi with pi function
eval('$result = '.$test.';');
}
else
{
$result = false;
}
?>
I can't guarantee you absolutely that this will block every possible malicious code nor that it will block malformed code, but that's better than the matheval function below which will allow malformed code like '2+2+' which will throw an error.
catgirl at charuru dot moe ¶
7 years ago
It should be noted that imported namespaces are not available in eval.
darkhogg (foo) gmail (bar) com ¶
14 years ago
The following code
<?php
eval( '?> foo <?php' );
?>
does not throw any error, but prints the opening tag.
Adding a space after the open tag fixes it:
<?php
eval( '?> foo <?php ' );
?>
remindfwd ¶
3 years ago
Note that
<?php
echo eval( '$var = (20 - 5);' ); // don't show anything
echo ' someString ' . eval( 'echo $var = 15;' ); // outputs 15 someString
//or
echo ' someString ' . eval( 'echo $var = 15;' ) . ' otherString '; // 15 someString otherString
//or
echo ' someString ' . eval( 'echo $var = 15;' ) . ' otherString ' . '...' .eval( 'echo " __ " . $var = 10;' ); // 15 __ 10 someString otherString ...
?>
divinity76 at gmail dot com ¶
7 years ago
imo, this is a better eval replacement:
<?php
function betterEval($code) {
$tmp = tmpfile ();
$tmpf = stream_get_meta_data ( $tmp );
$tmpf = $tmpf ['uri'];
fwrite ( $tmp, $code );
$ret = include ($tmpf);
fclose ( $tmp );
return $ret;
}
?>
- why? betterEval follows normal php opening and closing tag conventions, there's no need to strip `<?php?>` from the source. and it always throws a ParseError if there was a parse error, instead of returning false (note: this was fixed for normal eval() in php 7.0). - and there's also something about exception backtraces
xxixxek at gmail dot com ¶
1 year ago
I happened to work on a very old code that, for many reasons, couldn't be rewritten and the only way of showing the exact error in eval that worked for me was:
$res = eval($somecode);
if(!$res) {
echo "<pre>";
print_r(explode(PHP_EOL, $somecode));
echo "</pre>";
}
I know it is terrible but I didn't have much of a choice. None of the try...catch solutions worked for me; the solution above shows the exact lines with numbers and it is easy to find what's wrong with the code.
xxixxek at gmail dot com ¶
1 year ago
I happened to work on a very old code that, for many reasons, couldn't be rewritten and the only way of showing the exact error in eval that worked for me was:
$res = eval($somecode);
if(!$res) {
echo "<pre>";
print_r(explode(PHP_EOL, $somecode));
echo "</pre>";
}
I know it is terrible but I didn't have much of a choice. None of the try...catch solutions worked for me; the solution above shows the exact lines with numbers and it is easy to find what's wrong with the code.
stocki dot r at gmail dot com ¶
2 years ago
You can use `eval()` to combine classes/traits dynamically with anonymus classes:
<?php
function init($trait, $class) {
return (trait_exists($trait) && class_exists($class))
? eval("return new class() extends {$class} { use {$trait}; };")
: false;
}
trait Edit {
function hello() { echo 'EDIT: ' . $this->modulename; }
}
trait Ajax {
function hello() { echo 'AJAX: ' . $this->modulename; }
}
class MyModule {
public $modulename = 'My Module';
}
class AnotherModule {
public $modulename = 'Another Module';
}
init('Edit', 'MyModule')->hello(); # 'EDIT: My Module'
init('Ajax', 'AnotherModule')->hello(); # 'AJAX: Another Module'
?>
Karel ¶
9 years ago
For them who are facing syntax error when try execute code in eval,
<?php
$str = '<?php echo "test"; ?>';
eval('?>'.$str.'<?php;'); // outputs test
eval('?>'.$str.'<?'); // outputs test
eval('?>'.$str.'<?php');// throws syntax error - unexpected $end
?>
solobot ¶
7 years ago
eval() is workaround for generating multiple anonymous classes with static properties in loop
public function generateClassMap()
{
foreach ($this->classMap as $tableName => $class)
{
$c = null;
eval('$c = new class extends \common\MyStaticClass {
public static $tableName;
public static function tableName()
{
return static::$tableName;
}
};');
$c::$tableName = $this->replicationPrefix.$tableName;
$this->classMap[$tableName] = $c;
}
}
thus every class will have its own $tableName instead of common ancestor.
php at rijkvanwel dot nl ¶
14 years ago
To catch a parse error in eval()'ed code with a custom error handler, use error_get_last() (PHP >= 5.2.0).
<?php
$return = eval( 'parse error' );
if ( $return === false && ( $error = error_get_last() ) ) {
myErrorHandler( $error['type'], $error['message'], $error['file'], $error['line'], null );
// Since the "execution of the following code continues normally", as stated in the manual,
// we still have to exit explicitly in case of an error
exit;
}
?>
Uther ¶
8 years ago
eval'd code within namespaces which contain class and/or function definitions will be defined in the global namespace... not incredibly obvious :/
php at stock-consulting dot com ¶
16 years ago
Magic constants like __FILE__ may not return what you expect if used inside eval()'d code. Instead, it'll answer something like "c:\directory\filename.php(123) : eval()'d code" (under Windows, obviously, checked with PHP5.2.6) - which can still be processed with a function like preg_replace to receive the filename of the file containing the eval().
Example:
<?php
$filename = preg_replace('@\(.*\(.*$@', '', __FILE__);
echo $filename;
?>
greald at gmail dot com ¶
2 years ago
to avoid the evil eval() you may use the fact that function names, variable names, property names and method names can be handled strings.
<?php
class Fruit
{
public $tomato = "Tomatos";
public function red() {return " are red. ";}
}
$fruit = new Fruit;
$fruitStr = "tomato";
$colorStr = "red";
echo $fruit->$fruitStr . $fruit->$colorStr();
// and procedural //////////////////////////////////////////
$lemon = "Lemons";
function yellow() {return " are yellow. ";}
$fruitStr = "$lemon";
$colorStr = "yellow";
echo $fruitStr . $colorStr();
?>
Patanjali ¶
2 years ago
eval() is useful for preprocessing css (and js) with php to embed directly into a style tag in the head tag (or script tag at the bottom of body tag) of the HTML of the page.
This:
a. Prevents Flash of White in Chrome or Firefox (where an external css file arrives briefly too late to render the HTML).
b. Allows radical minifying by testing the page source to see if whole blocks of rules or code are even required, such as for tables.
c. Allows custom source-content-dependent css rules to be created on the fly. (I use this to create rules for positioned labels over an image that scale with it)
d. Allows generation of a hash of the processed css or js for use in the page's CSP header for style-src or script-src to prevent injection attacks.
Here eval() is safe because it is not using user-supplied (person or browser) information
Ipseno at yahoo dot com ¶
16 years ago
If you attempt to call a user defined function in eval() and .php files are obfuscated by Zend encoder, it will result in a fatal error.
Use a call_user_func() inside eval() to call your personal hand made functions.
This is user function
<?php
function square_it($nmb)
{
return $nmb * $nmb;
}
?>
//Checking if eval sees it?
<?php
$code = var_export( function_exists('square_it') );
eval( $code ); //returns TRUE - so yes it does!
?>
This will result in a fatal error:
PHP Fatal error: Call to undefined function square_it()
<?php
$code = 'echo square_it(55);' ;
eval( $code );
?>
This will work
<?php
$code = 'echo call_user_func(\'square_it\', 55);' ;
eval( $code );
?>
marco at harddisk dot is-a-geek dot org ¶
16 years ago
eval does not work reliably in conjunction with global, at least not in the cygwin port version.
So:
<?PHP
class foo {
//my class...
}
function load_module($module) {
eval("global \$".$module."_var;");
eval("\$".$module."_var=&new foo();");
//various stuff ... ...
}
load_module("foo");
?>
becomes to working:
<?PHP
class foo {
//my class...
}
function load_module($module) {
eval('$GLOBALS["'.$module.'_var"]=&new foo();');
//various stuff ... ...
}
load_module("foo");
?>
Note in the 2nd example, you _always_ need to use $GLOBALS[$module] to access the variable!
