The ExpressionLanguage Component
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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
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$ composer require symfony/expression-language
Alternatively, you can clone the https://github.com/symfony/expression-language repository.
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:
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# 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 ExpressionLanguage:
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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):
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use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
$expressionLanguage = new ExpressionLanguage();
class Apple
{
public $variety;
}
$apple = new Apple();
$apple->variety = 'Honeycrisp';
var_dump($expressionLanguage->evaluate(
'fruit.variety',
array(
'fruit' => $apple,
)
));
This will print "Honeycrisp". For more information, see the The Expression Syntax entry, especially The Expression Syntax and The Expression Syntax.
Caching
The component provides some different caching strategies, read more about them in Caching Expressions Using Parser Caches.