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What Is PHP AST?

An AST (Abstract Syntax Tree) is a tree structure that decomposes source code into meaningful syntax structures. When you build code generation tools, static analysis tools, or automated refactoring tools, AST is almost always a required foundation. Even transformations that are fragile with string replacement can be handled safely at syntax-unit level with AST, such as function calls, class declarations, and use statements.

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PHP Internal AST

Since PHP 7, the Zend Engine compiles PHP code by first transforming it into an internal AST, then compiling it into opcodes.
  • Even one-line code executed with php -r goes through the same internal compilation pipeline
  • OPcache caches and reuses the resulting opcodes
  • In normal application development, you rarely manipulate this internal AST directly
So when you want to edit code via AST, the practical approach is to use a userland AST library rather than the internal runtime AST.

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The nikic/PHP-Parser Package

nikic/PHP-Parser is the standard library for parsing, traversing, and regenerating PHP code as AST. Its README and official docs describe this three-step flow as the baseline.

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Installation

composer require nikic/php-parser

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Basic Parsing Example (Code to AST)

<?php

use PhpParser\Error;
use PhpParser\ParserFactory;

$code = <<<'CODE'
<?php
function greet(string $name): void {
    echo "Hello, {$name}";
}
CODE;

$parser = (new ParserFactory())->createForNewestSupportedVersion();

try {
    $ast = $parser->parse($code);
} catch (Error $error) {
    echo "Parse error: {$error->getMessage()}\n";
    return;
}

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Traverse and Modify AST with the NodeVisitor Pattern

<?php

use PhpParser\Node;
use PhpParser\NodeTraverser;
use PhpParser\NodeVisitorAbstract;

$traverser = new NodeTraverser();

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

$modifiedAst = $traverser->traverse($ast);
When you extend NodeVisitorAbstract, you can implement only the hooks you need (enterNode / leaveNode). For complex transforms, a common flow is collecting context in enterNode and replacing nodes in leaveNode.

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Regenerate Code with Pretty Printer

<?php

use PhpParser\PrettyPrinter;

$prettyPrinter = new PrettyPrinter\Standard();
$newCode = $prettyPrinter->prettyPrintFile($modifiedAst);

echo $newCode;
With this flow, your target becomes code as syntax trees, not code as plain strings.

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Usage in Laravel/Chisel

laravel/chisel is a library that removes optional starter-kit parts in a post-processing step. In composer.json, it depends on nikic/php-parser 5.x. In Chisel’s Laravel\Chisel\Ast\Source, AST editing is executed with this sequence.
  1. Parse source with ParserFactory::createForNewestSupportedVersion()
  2. Add multiple visitors (such as RemoveImportVisitor) to NodeTraverser and transform
  3. Write back with PhpParser\PrettyPrinter\Standard using printFormatPreserving() to preserve original formatting
With this design, removal of use statements, traits, and interfaces can be done safely as syntax operations rather than text replacement.

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Use Cases

AST makes the following development tasks easier.
  • Code generation CLI (syntax-aware edits after scaffolding)
  • Static analysis tools (detecting specific syntax and rule violations)
  • Automated refactoring (semi-automated API migration and renaming)
  • Post-processing project templates (feature removal/replacement like Chisel)

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Summary

PHP AST is not a feature you use every day in typical Laravel app development. However, if you build tools or packages, AST is a strong foundation for resilient and reproducible code changes. Start by trying the Parser, Visitor, and Pretty Printer of nikic/php-parser in a small CLI tool.
Last modified on May 17, 2026