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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.

PHPStan uses the popular PHP Parser library from Nikita Popov to obtain and traverse the AST.

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_ (return keyword)
  • PhpParser\Node\Stmt\Switch_ (switch keyword)
  • PhpParser\Node\Stmt\For_ (for loop)

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 instanceof keyword, type is either true or false)

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.

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