xdebug/blog/vendor/nikic/php-parser/doc/component/Lexer.markdown

Lexer component documentation

The lexer is responsible for providing tokens to the parser. The project comes with two lexers: PhpParser\Lexer and PhpParser\Lexer\Emulative. The latter is an extension of the former, which adds the ability to emulate tokens of newer PHP versions and thus allows parsing of new code on older versions.

This documentation discusses options available for the default lexers and explains how lexers can be extended.

Lexer options

The two default lexers accept an $options array in the constructor. Currently only the 'usedAttributes' option is supported, which allows you to specify which attributes will be added to the AST nodes. The attributes can then be accessed using $node->getAttribute(), $node->setAttribute(), $node->hasAttribute() and $node->getAttributes() methods. A sample options array:

$lexer = new PhpParser\Lexer(array(
    'usedAttributes' => array(
        'comments', 'startLine', 'endLine'
    )
));

The attributes used in this example match the default behavior of the lexer. The following attributes are supported:

• comments: Array of PhpParser\Comment or PhpParser\Comment\Doc instances, representing all comments that occurred between the previous non-discarded token and the current one. Use of this attribute is required for the $node->getDocComment() method to work. The attribute is also needed if you wish the pretty printer to retain comments present in the original code.
• startLine: Line in which the node starts. This attribute is required for the $node->getLine() to work. It is also required if syntax errors should contain line number information.
• endLine: Line in which the node ends.
• startTokenPos: Offset into the token array of the first token in the node.
• endTokenPos: Offset into the token array of the last token in the node.
• startFilePos: Offset into the code string of the first character that is part of the node.
• endFilePos: Offset into the code string of the last character that is part of the node.

Using token positions

The token offset information is useful if you wish to examine the exact formatting used for a node. For example the AST does not distinguish whether a property was declared using public or using var, but you can retrieve this information based on the token position:

function isDeclaredUsingVar(array $tokens, PhpParser\Node\Stmt\Property $prop) {
    $i = $prop->getAttribute('startTokenPos');
    return $tokens[$i][0] === T_VAR;
}

In order to make use of this function, you will have to provide the tokens from the lexer to your node visitor using code similar to the following:

class MyNodeVisitor extends PhpParser\NodeVisitorAbstract {
    private $tokens;
    public function setTokens(array $tokens) {
        $this->tokens = $tokens;
    }

    public function leaveNode(PhpParser\Node $node) {
        if ($node instanceof PhpParser\Node\Stmt\Property) {
            var_dump(isDeclaredUsingVar($this->tokens, $node));
        }
    }
}

$lexer = new PhpParser\Lexer(array(
    'usedAttributes' => array(
        'comments', 'startLine', 'endLine', 'startTokenPos', 'endTokenPos'
    )
));
$parser = (new PhpParser\ParserFactory)->create(PhpParser\ParserFactory::PREFER_PHP7, $lexer);

$visitor = new MyNodeVisitor();
$traverser = new PhpParser\NodeTraverser();
$traverser->addVisitor($visitor);

try {
    $stmts = $parser->parse($code);
    $visitor->setTokens($lexer->getTokens());
    $stmts = $traverser->traverse($stmts);
} catch (PhpParser\Error $e) {
    echo 'Parse Error: ', $e->getMessage();
}

The same approach can also be used to perform specific modifications in the code, without changing the formatting in other places (which is the case when using the pretty printer).

Lexer extension

A lexer has to define the following public interface:

void startLexing(string $code, ErrorHandler $errorHandler = null);
array getTokens();
string handleHaltCompiler();
int getNextToken(string &$value = null, array &$startAttributes = null, array &$endAttributes = null);

The startLexing() method is invoked with the source code that is to be lexed (including the opening tag) whenever the parse() method of the parser is called. It can be used to reset state or preprocess the source code or tokens. The passes ErrorHandler should be used to report lexing errors.

The getTokens() method returns the current token array, in the usual token_get_all() format. This method is not used by the parser (which uses getNextToken()), but is useful in combination with the token position attributes.

The handleHaltCompiler() method is called whenever a T_HALT_COMPILER token is encountered. It has to return the remaining string after the construct (not including ();).

The getNextToken() method returns the ID of the next token (as defined by the Parser::T_* constants). If no more tokens are available it must return 0, which is the ID of the EOF token. Furthermore the string content of the token should be written into the by-reference $value parameter (which will then be available as $n in the parser).

Attribute handling

The other two by-ref variables $startAttributes and $endAttributes define which attributes will eventually be assigned to the generated nodes: The parser will take the $startAttributes from the first token which is part of the node and the $endAttributes from the last token that is part of the node.

E.g. if the tokens T_FUNCTION T_STRING ... '{' ... '}' constitute a node, then the $startAttributes from the T_FUNCTION token will be taken and the $endAttributes from the '}' token.

An application of custom attributes is storing the exact original formatting of literals: While the parser does retain some information about the formatting of integers (like decimal vs. hexadecimal) or strings (like used quote type), it does not preserve the exact original formatting (e.g. leading zeros for integers or escape sequences in strings). This can be remedied by storing the original value in an attribute:

use PhpParser\Lexer;
use PhpParser\Parser\Tokens;

class KeepOriginalValueLexer extends Lexer // or Lexer\Emulative
{
    public function getNextToken(&$value = null, &$startAttributes = null, &$endAttributes = null) {
        $tokenId = parent::getNextToken($value, $startAttributes, $endAttributes);

        if ($tokenId == Tokens::T_CONSTANT_ENCAPSED_STRING   // non-interpolated string
            || $tokenId == Tokens::T_ENCAPSED_AND_WHITESPACE // interpolated string
            || $tokenId == Tokens::T_LNUMBER                 // integer
            || $tokenId == Tokens::T_DNUMBER                 // floating point number
        ) {
            // could also use $startAttributes, doesn't really matter here
            $endAttributes['originalValue'] = $value;
        }

        return $tokenId;
    }
}