Master Symfony ExpressionLanguage: Install, Use, and Extend with Custom Functions

Installation

Install the component via Composer: composer require symfony/expression-language If the component is used outside a Symfony application, include vendor/autoload.php to enable Composer's autoloader.

Why Use ExpressionLanguage?

The component lets you write expressions in configuration files to implement complex logic such as security rules, validation constraints, and route matching. It also serves as a lightweight business‑rule engine, allowing non‑PHP administrators to configure behavior safely.

Basic Usage

ExpressionLanguage can evaluate an expression directly or compile it into PHP code for caching.

<?php
require '../vendor/autoload.php';
$el = new \Symfony\Component\ExpressionLanguage\ExpressionLanguage();
var_dump($el->evaluate('1 + 2')); // displays 3
var_dump($el->compile('1 + 2'));   // displays (1 + 2)
?>

Parsing and Linting

The parse() method returns a ParsedExpression object that represents the internal structure of an expression. Symfony does not provide a dedicated lint() method; instead you can attempt to parse the expression and catch a SyntaxError to detect invalid syntax.

<?php
require '../vendor/autoload.php';
use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
$el = new ExpressionLanguage();
try {
    $el->lint('1 + 2 * (3 - 4');
    echo "Expression syntax is correct
";
} catch (\Symfony\Component\ExpressionLanguage\SyntaxError $e) {
    echo "Syntax error: " . $e->getMessage() . "
";
}
?>

Registering Custom Functions

Functions are registered per ExpressionLanguage instance using register(), which accepts a name, a compiler callback, and an evaluator callback.

<?php
require '../vendor/autoload.php';
$el = new \Symfony\Component\ExpressionLanguage\ExpressionLanguage();
$el->register('custom_function',
    function ($str) { return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str); },
    function ($args, $str) { return is_string($str) ? strtolower($str) : $str; }
);
var_dump($el->evaluate('custom_function("HELLO")'));
?>

The evaluator receives an arguments array as its first parameter, containing the values passed to evaluate().

Objects and Their Properties

Expressions can access PHP objects and their properties directly.

<?php
require '../vendor/autoload.php';
use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
$el = new ExpressionLanguage();
class Product { public $name; public $price; }
$p = new Product();
$p->name = 'Tinywan';
$p->price = 2024;
echo $el->evaluate('product.price', ['product' => $p]) . "
";
echo $el->evaluate('product.price > 2022', ['product' => $p]) . "
";
?>

Product price is 2024
Is Product price higher than 2022: 1

Other Applications

The component can be integrated with access‑control libraries such as PHP‑Casbin. Example code shows how to create an ExpressionLanguage instance, register functions, parse a policy expression, and evaluate it to obtain an allow/deny decision.

protected function getExpressionLanguage(array $functions): ExpressionLanguage {
    $el = new ExpressionLanguage();
    foreach ($functions as $key => $func) {
        $el->register($key,
            function (...$args) use ($key) { return sprintf($key.'(%s)', implode(',', $args)); },
            function ($arguments, ...$args) use ($func) { return $func(...$args); }
        );
    }
    return $el;
}
$el = $this->getExpressionLanguage($functions);
$expression = $el->parse($expWithRule, array_merge($rTokens, $pTokens));
$result = $el->evaluate($expression, $parameters);
if ($result) { $policyEffects[0] = Effector::ALLOW; } else { $policyEffects[0] = Effector::INDETERMINATE; }