PHP-Parser/doc/component/Walking_the_AST.markdown

Walking the AST

The most common way to work with the AST is by using a node traverser and one or more node visitors. As a basic example, the following code changes all literal integers in the AST into strings (e.g., 42 becomes '42'.)

use PhpParser\{Node, NodeTraverser, NodeVisitorAbstract};

$traverser = new NodeTraverser;
$traverser->addVisitor(new class extends NodeVisitorAbstract {
    public function leaveNode(Node $node) {
        if ($node instanceof Node\Scalar\LNumber) {
            return new Node\Scalar\String_((string) $node->value);
        }
    }
});

$stmts = ...;
$modifiedStmts = $traverser->traverse($stmts);

Node visitors

Each node visitor implements an interface with following four methods:

interface NodeVisitor {
    public function beforeTraverse(array $nodes);
    public function enterNode(Node $node);
    public function leaveNode(Node $node);
    public function afterTraverse(array $nodes);
}

The beforeTraverse() and afterTraverse() methods are called before and after the traversal respectively, and are passed the entire AST. They can be used to perform any necessary state setup or cleanup.

The enterNode() method is called when a node is first encountered, before its children are processed ("preorder"). The leaveNode() method is called after all children have been visited ("postorder").

For example, if we have the following excerpt of an AST

Expr_FuncCall(
    name: Name(
        parts: array(
            0: printLine
        )
    )
    args: array(
        0: Arg(
            value: Scalar_String(
                value: Hello World!!!
            )
            byRef: false
            unpack: false
        )
    )
)

then the enter/leave methods will be called in the following order:

enterNode(Expr_FuncCall)
enterNode(Name)
leaveNode(Name)
enterNode(Arg)
enterNode(Scalar_String)
leaveNode(Scalar_String)
leaveNode(Arg)
leaveNode(Expr_FuncCall)

A common pattern is that enterNode is used to collect some information and then leaveNode performs modifications based on that. At the time when leaveNode is called, all the code inside the node will have already been visited and necessary information collected.

As you usually do not want to implement all four methods, it is recommended that you extend NodeVisitorAbstract instead of implementing the interface directly. The abstract class provides empty default implementations.

Modifying the AST

There are a number of ways in which the AST can be modified from inside a node visitor. The first and simplest is to simply change AST properties inside the visitor:

public function leaveNode(Node $node) {
    if ($node instanceof Node\Scalar\LNumber) {
        // increment all integer literals
        $node->value++;
    }
}

The second is to replace a node entirely by returning a new node:

public function leaveNode(Node $node) {
    if ($node instanceof Node\Expr\BinaryOp\BooleanAnd) {
        // Convert all $a && $b expressions into !($a && $b)
        return new Node\Expr\BooleanNot($node);
    }
}

Doing this is supported both inside enterNode and leaveNode. However, you have to be mindful about where you perform the replacement: If a node is replaced in enterNode, then the recursive traversal will also consider the children of the new node. If you aren't careful, this can lead to infinite recursion. For example, let's take the previous code sample and use enterNode instead:

public function enterNode(Node $node) {
    if ($node instanceof Node\Expr\BinaryOp\BooleanAnd) {
        // Convert all $a && $b expressions into !($a && $b)
        return new Node\Expr\BooleanNot($node);
    }
}

Now $a && $b will be replaced by !($a && $b). Then the traverser will go into the first (and only) child of !($a && $b), which is $a && $b. The transformation applies again and we end up with !!($a && $b). This will continue until PHP hits the memory limit.

Finally, two special replacement types are supported only by leaveNode. The first is removal of a node:

public function leaveNode(Node $node) {
    if ($node instanceof Node\Stmt\Return_) {
        // Remove all return statements
        return NodeTraverser::REMOVE_NODE;
    }
}

Node removal only works if the parent structure is an array. This means that usually it only makes sense to remove nodes of type Node\Stmt, as they always occur inside statement lists (and a few more node types like Arg or Expr\ArrayItem, which are also always part of lists).

On the other hand, removing a Node\Expr does not make sense: If you have $a * $b, there is no meaningful way in which the $a part could be removed. If you want to remove an expression, you generally want to remove it together with a surrounding expression statement:

public function leaveNode(Node $node) {
    if ($node instanceof Node\Stmt\Expression
        && $node->expr instanceof Node\Expr\FuncCall
        && $node->expr->name instanceof Node\Name
        && $node->expr->name->toString() === 'var_dump'
    ) {
        return NodeTraverser::REMOVE_NODE;
    }
}

This example will remove all calls to var_dump() which occur as expression statements. This means that var_dump($a); will be removed, but if (var_dump($a)) will not be removed (and there is no obvious way in which it can be removed).

Next to removing nodes, it is also possible to replace one node with multiple nodes. Again, this only works inside leaveNode and only if the parent structure is an array.

public function leaveNode(Node $node) {
    if ($node instanceof Node\Stmt\Return_ && $node->expr !== null) {
        // Convert "return foo();" into "$retval = foo(); return $retval;"
        $var = new Node\Expr\Variable('retval');
        return [
            new Node\Stmt\Expression(new Node\Expr\Assign($var, $node->expr)),
            new Node\Stmt\Return_($var),
        ];
    }
}

Short-circuiting traversal

An AST can easily contain thousands of nodes, and traversing over all of them may be slow, especially if you have more than one visitor. In some cases, it is possible to avoid a full traversal.

If you are looking for all class declarations in a file (and assuming you're not interested in anonymous classes), you know that once you've seen a class declaration, there is no point in also checking all it's child nodes, because PHP does not allow nesting classes. In this case, you can instruct the traverser to not recurse into the class node:

private $classes = [];
public function enterNode(Node $node) {
    if ($node instanceof Node\Stmt\Class_) {
        $this->classes[] = $node;
        return NodeTraverser::DONT_TRAVERSE_CHILDREN;
    }
}

Of course, this option is only available in enterNode, because it's already too late by the time leaveNode is reached.

If you are only looking for one specific node, it is also possible to abort the traversal entirely after finding it. For example, if you are looking for the node of a class with a certain name (and discounting exotic cases like conditionally defining a class two times), you can stop traversal once you found it:

private $class = null;
public function enterNode(Node $node) {
    if ($node instanceof Node\Stmt\Class_ &&
        $node->namespaceName->toString() === 'Foo\Bar\Baz'
    ) {
        $this->class = $node;
        return NodeTraverser::STOP_TRAVERSAL;
    }
}

This works both in enterNode and leaveNode. Note that this particular case can also be more easily handled using a NodeFinder, which will be introduced below.

Multiple visitors

A single traverser can be used with multiple visitors:

$traverser = new NodeTraverser;
$traverser->addVisitor($visitorA);
$traverser->addVisitor($visitorB);
$stmts = $traverser->traverser($stmts);

It is important to understand that if a traverser is run with multiple visitors, the visitors will be interleaved. Given the following AST excerpt

Stmt_Return(
    expr: Expr_Variable(
        name: foobar
    )
)

the following method calls will be performed:

$visitorA->enterNode(Stmt_Return)
$visitorB->enterNode(Stmt_Return)
$visitorA->enterNode(Expr_Variable)
$visitorB->enterNode(Expr_Variable)
$visitorA->leaveNode(Expr_Variable)
$visitorB->leaveNode(Expr_Variable)
$visitorA->leaveNode(Stmt_Return)
$visitorB->leaveNode(Stmt_Return)

That is, when visiting a node, enterNode and leaveNode will always be called for all visitors. Running multiple visitors in parallel improves performance, as the AST only has to be traversed once. However, it is not always possible to write visitors in a way that allows interleaved execution. In this case, you can always fall back to performing multiple traversals:

$traverserA = new NodeTraverser;
$traverserA->addVisitor($visitorA);
$traverserB = new NodeTraverser;
$traverserB->addVisitor($visitorB);
$stmts = $traverserA->traverser($stmts);
$stmts = $traverserB->traverser($stmts);

When using multiple visitors, it is important to understand how they interact with the various special enterNode/leaveNode return values:

• If any visitor returns DONT_TRAVERSE_CHILDREN, the children will be skipped for all visitors.
• If any visitor returns STOP_TRAVERSAL, traversal is stopped for all visitors.
• If a visitor returns a replacement node, subsequent visitors will be passed the replacement node, not the original one.
• If a visitor returns REMOVE_NODE, subsequent visitors will not see this node.
• If a visitor returns an array of replacement nodes, subsequent visitors will see neither the node that was replaced, nor the replacement nodes.

Simple node finding

While the node visitor mechanism is very flexible, creating a node visitor can be overly cumbersome for minor tasks. For this reason a NodeFinder is provided, which can find AST nodes that either satisfy a certain callback, or which are instanced of a certain node type. A couple of examples are shown in the following:

use PhpParser\{Node, NodeFinder};

$nodeFinder = new NodeFinder;

// Find all class nodes.
$classes = $nodeFinder->findInstanceOf($stmts, Node\Stmt\Class_::class);

// Find all classes that extend another class
$extendingClasses = $nodeFinder->findInstanceOf($stmts, function(Node $node) {
    return $node instanceof Node\Stmt\Class_
        && $node->extends !== null;
});

// Find first class occuring in the AST. Returns null if no class exists.
$class = $nodeFinder->findFirstInstanceOf($stmts, Node\Stmt\Class_::class);

// Find first class that has name $name
$class = $nodeFinder->findFirst($stmts, function(Node $node) use ($name) {
    return $node instanceof Node\Stmt\Class_
        && $node->resolvedName->toString() === $name;
});

Internally, the NodeFinder also uses a node traverser. It only simplifies the interface for a common use case.

Parent and sibling references

The node visitor mechanism is somewhat rigid, in that it prescribes an order in which nodes should be accessed: From parents to children. However, it can often be convenient to operate in the reverse direction: When working on a node, you might want to check if the parent node satisfies a certain property.

PHP-Parser does not add parent (or sibling) references to nodes by itself, but you can easily emulate this with a visitor. See the FAQ for more information.