Abstract Syntax Tree
Abstract Syntax Tree (AST for short) is the way analysed source code is represented in the static analyser so that it can be queried for useful information.
AST represents the parsed source code as a hierarchy of objects. Let’s say we have the following PHP code:
return $this->foo && self::bar();
This is how the AST for this code looks like:
PhpParser\Node\Stmt\Return_ └─ expr: PhpParser\Node\Expr\BinaryOp\BooleanAnd ├─ left: PhpParser\Node\Expr\PropertyFetch | ├─ var: PhpParser\Node\Expr\Variable | └─ name: PhpParser\Node\Identifier └─ right: PhpParser\Node\Expr\StaticCall ├─ class: PhpParser\Node\Name ├─ name: PhpParser\Node\Identifier └─ args: array()
The PHP Parser library contains dozens of node types for all possible constructs that can be written in PHP code.
The AST doesn’t care how is the code formatted - it throws away information about whitespace, and some extra parentheses.
Statements vs. expressions #
There are two main categories of nodes in the AST: statements and expressions.
Statements are usually controlling the flow of the program, and declare new symbols. They usually come with their own keyword. Statements themselves cannot have a type. Examples of statements are:
- PhpParser\Node\Stmt\Return_ (
- PhpParser\Node\Stmt\Switch_ (
- PhpParser\Node\Stmt\For_ (
Expressions usually consist of other expressions, and can be resolved to a type. Examples of expressions are:
- PhpParser\Node\Expr\Variable (variable like
$foo, type is the type of the variable)
- PhpParser\Node\Expr\PropertyFetch (accessing a property:
$this->foo, type is the type of the property)
- PhpParser\Node\Expr\Instanceof_ (the
instanceofkeyword, type is either
To retrieve the type of an expression, you need to call the
getType() method on the Scope object. You’ll obtain an object implementing the
PHPStan\Type\Type interface. See the article about the type system to learn how to use it.