Kepp the following Quote in mind:
If eval() is the answer, you're almost certainly asking the
wrong question. -- Rasmus Lerdorf, BDFL of PHP
(PHP 4, PHP 5, PHP 7, PHP 8)
eval — Executa uma string como código PHP
A função eval() executa a string dada no parâmetro
code_str como código PHP. Entre outras coisas,
isto pode ser útil para guardar código em um campo de texto de um banco de dados para execução posterior.
Há alguns fatores para lembrar quando usar
eval(). Lembre-se que a string passada deve ser código
PHP valido, incluindo coisas como terminar os comandos com ponto-e-vírgula
assim o compilador não morre uma linha após
eval(), escapar as coisas propriamente em
code_str.
Também se lembre que as variáveis que tenham seus valores em eval() irão reter estes valores para o resto do script.
Um comando return irá terminar a execução do script imediatamente
No PHP 4, eval() retorna
null a menos que return seja chamado no código executado,
caso no qual o valor passado para return é retornado.
No PHP 3, eval() não retorna um valor.
Exemplo #1 Exemplo eval()
<?php
$string = 'taça';
$name = 'café';
$str = 'Esta é uma $string com o meu $name nela.';
echo $str. "\n";
eval("\$str = \"$str\";");
echo $str . "\n";
?>
O exemplo acima irá mostrar
Esta é uma $string com o meu $name nela. Esta é uma taça com o meu café nela.
Como toda saída é normalmente enviada direto para o browser, as Funções de Controle de Output podem ser utilizadas para capturar o resultado e guardá-lo em uma string (por exemplo).
Veja também call_user_func().
Kepp the following Quote in mind:
If eval() is the answer, you're almost certainly asking the
wrong question. -- Rasmus Lerdorf, BDFL of PHP
Note that
<?php
echo eval( '$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 " __ " . $var = 10;' ); // 15 __ 10 someString otherString ...
?>
At least in PHP 7.1+, eval() terminates the script if the evaluated code generate a fatal error. For example:
<?php
@eval('$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) {
$content = null;
}
?>
This is the only way I found to catch the error and hide the fact there was one.
Inception with eval()
<pre>
Inception Start:
<?php
eval("echo 'Inception lvl 1...\n'; eval('echo \"Inception lvl 2...\n\"; eval(\"echo \'Inception lvl 3...\n\'; eval(\'echo \\\"Limbo!\\\";\');\");');");
?>
You can use `eval()` to combine classes/traits dynamically with anonymus classes:
<?php
function init($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'
?>
It should be noted that imported namespaces are not available in eval.
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
{
$result = false;
}
?>
I can't guarantee you absolutely that this will block every possible malicious code nor that it will block malformed code, but that's better than the matheval function below which will allow malformed code like '2+2+' which will throw an error.
eval() is workaround for generating multiple anonymous classes with static properties in loop
public function generateClassMap()
{
foreach ($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 $tableName instead of common ancestor.
The following code
<?php
eval( '?> foo <?php' );
?>
does not throw any error, but prints the opening tag.
Adding a space after the open tag fixes it:
<?php
eval( '?> foo <?php ' );
?>
imo, this is a better eval replacement:
<?php
function betterEval($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
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.
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.
eval'd code within namespaces which contain class and/or function definitions will be defined in the global namespace... not incredibly obvious :/
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
?>
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;
}
?>
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;
?>
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 . $colorStr();
?>
eval() is useful for preprocessing css (and js) with php to embed directly into a style tag in the head tag (or script tag at the bottom of body tag) of the HTML of the page.
This:
a. Prevents Flash of White in Chrome or Firefox (where an external css file arrives briefly too late to render the HTML).
b. Allows radical minifying by testing the page source to see if whole blocks of rules or code are even required, such as for tables.
c. Allows custom source-content-dependent css rules to be created on the fly. (I use this to create rules for positioned labels over an image that scale with it)
d. Allows generation of a hash of the processed css or js for use in the page's CSP header for style-src or script-src to prevent injection attacks.
Here eval() is safe because it is not using user-supplied (person or browser) information
eval does not work reliably in conjunction with global, at least not in the cygwin port version.
So:
<?PHP
class foo {
//my class...
}
function load_module($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!
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 );
?>