The ExpressionLanguage Component

The ExpressionLanguage Component

The ExpressionLanguage Component

The ExpressionLanguage component provides an engine that can compile and evaluate expressions. An expression is a one-liner that returns a value (mostly, but not limited to, Booleans).

Installation

$ composer require symfony/expression-language

Note

If you install this component outside of a Symfony application, you must require the vendor/autoload.php file in your code to enable the class autoloading mechanism provided by Composer. Read this article for more details.

How can the Expression Engine Help Me?

The purpose of the component is to allow users to use expressions inside configuration for more complex logic. For some examples, the Symfony Framework uses expressions in security, for validation rules and in route matching.

Besides using the component in the framework itself, the ExpressionLanguage component is a perfect candidate for the foundation of a business rule engine. The idea is to let the webmaster of a website configure things in a dynamic way without using PHP and without introducing security problems:

# Get the special price if
user.getGroup() in ['good_customers', 'collaborator']
# Promote article to the homepage when
article.commentCount > 100 and article.category not in ["misc"]
# Send an alert when
product.stock < 15

Expressions can be seen as a very restricted PHP sandbox and are immune to external injections as you must explicitly declare which variables are available in an expression.

Usage

The ExpressionLanguage component can compile and evaluate expressions. Expressions are one-liners that often return a Boolean, which can be used by the code executing the expression in an if statement. A simple example of an expression is 1 + 2. You can also use more complicated expressions, such as `someArray[3].someMethod(‘bar’).

The component provides 2 ways to work with expressions:

evaluation: the expression is evaluated without being compiled to PHP;
compile: the expression is compiled to PHP, so it can be cached and evaluated.

The main class of the component is Symfony\Component\ExpressionLanguage\ExpressionLanguage:

use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
$expressionLanguage = new ExpressionLanguage();
var_dump($expressionLanguage->evaluate('1 + 2')); // displays 3
var_dump($expressionLanguage->compile('1 + 2')); // displays (1 + 2)

Expression Syntax

See The Expression Syntax to learn the syntax of the ExpressionLanguage component.

Passing in Variables

You can also pass variables into the expression, which can be of any valid PHP type (including objects):

use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
$expressionLanguage = new ExpressionLanguage();
class Apple
{
    public $variety;
}
$apple = new Apple();
$apple->variety = 'Honeycrisp';
var_dump($expressionLanguage->evaluate(
    'fruit.variety',
    [
        'fruit' => $apple,
    ]
)); // displays "Honeycrisp"

For more information, see the The Expression Syntax entry, especially Working with Objects and Working with Arrays.

Caution

When using variables in expressions, avoid passing untrusted data into the array of variables. If you can’t avoid that, sanitize non-alphanumeric characters in untrusted data to prevent malicious users from injecting control characters and altering the expression.

Caching

The component provides some different caching strategies, read more about them in Caching Expressions Using Parser Caches.

AST Dumping and Editing

The AST (Abstract Syntax Tree) of expressions can be dumped and manipulated as explained in Dumping and Manipulating the AST of Expressions.

Learn More

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.