PHP-Parser/doc/2_Usage_of_basic_components.markdown

Usage of basic components
=========================

This document explains how to use the parser, the pretty printer and the node traverser.

Bootstrapping
-------------

The library needs to register a class autoloader; this is done by including the
`bootstrap.php` file:

```php
<?php
require 'path/to/PHP-Parser/lib/bootstrap.php';
```

Additionally you may want to set the `xdebug.max_nesting_level` ini option to a higher value:

```php
<?php
ini_set('xdebug.max_nesting_level', 2000);
```

This ensures that there will be no errors when traversing highly nested node trees.

Parsing
-------

In order to parse some source code you first have to create a `PhpParser\Parser` object (which
needs to be passed a `PhpParser\Lexer` instance) and then pass the code (including `<?php` opening
tags) to the `parse` method. If a syntax error is encountered `PhpParser\Error` is thrown, so this
exception should be `catch`ed.

```php
<?php
$code = '<?php // some code';

$parser = new PhpParser\Parser(new PhpParser\Lexer);

try {
    $stmts = $parser->parse($code);
} catch (PhpParser\Error $e) {
    echo 'Parse Error: ', $e->getMessage();
}
```

The `parse` method will return an array of statement nodes (`$stmts`).

### Emulative lexer

Instead of `PhpParser\Lexer` one can also use `PhpParser\Lexer\Emulative`. This class will emulate tokens
of newer PHP versions and as such allow parsing PHP 5.5 on PHP 5.2, for example. So if you want to parse
PHP code of newer versions than the one you are running, you should use the emulative lexer.

Node tree
---------

If you use the above code with `$code = "<?php echo 'Hi ', hi\\getTarget();"` the parser will
generate a node tree looking like this:

```
array(
    0: Stmt_Echo(
        exprs: array(
            0: Scalar_String(
                value: Hi
            )
            1: Expr_FuncCall(
                name: Name(
                    parts: array(
                        0: hi
                        1: getTarget
                    )
                )
                args: array(
                )
            )
        )
    )
)
```

Thus `$stmts` will contain an array with only one node, with this node being an instance of
`PhpParser\Node\Stmt\Echo_`.

As PHP is a large language there are approximately 140 different nodes. In order to make work
with them easier they are grouped into three categories:

 * `PhpParser\Node\Stmt`s are statement nodes, i.e. language constructs that do not return
   a value and can not occur in an expression. For example a class definition is a statement.
   It doesn't return a value and you can't write something like `func(class A {});`.
 * `PhpParser\Node\Expr`s are expression nodes, i.e. language constructs that return a value
   and thus can occur in other expressions. Examples of expressions are `$var`
   (`PhpParser\Node\Expr\Variable`) and `func()` (`PhpParser\Node\Expr\FuncCall`).
 * `PhpParser\Node\Scalar`s are nodes representing scalar values, like `'string'`
   (`PhpParser\Node\Scalar\String`), `0` (`PhpParser\Node\Scalar\LNumber`) or magic constants
   like `__FILE__` (`PhpParser\Node\Scalar\MagicConst\File`). All `PhpParser\Node\Scalar`s extend
   `PhpParser\Node\Expr`, as scalars are expressions, too.
 * There are some nodes not in either of these groups, for example names (`PhpParser\Node\Name`)
   and call arguments (`PhpParser\Node\Arg`).

Some node class names have a trailing `_`. This is used whenever the class name would otherwise clash
with a PHP keyword.

Every node has a (possibly zero) number of subnodes. You can access subnodes by writing
`$node->subNodeName`. The `Stmt\Echo_` node has only one subnode `exprs`. So in order to access it
in the above example you would write `$stmts[0]->exprs`. If you wanted to access name of the function
call, you would write `$stmts[0]->exprs[1]->name`.

All nodes also define a `getType()` method that returns the node type. The type is the class name
without the `PHPParser\Node\` prefix and `\` replaced with `_`. It also does not contain a trailing
`_` for reserved-keyword class names.

It is possible to associate custom metadata with a node using the `setAttribute()` method. This data
can then be retrieved using `hasAttribute()`, `getAttribute()` and `getAttributes()`.

By default the lexer adds the `startLine`, `endLine` and `comments` attributes. `comments` is an array
of `PhpParser\Comment[\Doc]` instances.

The start line can also be accessed using `getLine()`/`setLine()` (instead of `getAttribute('startLine')`).
The last doc comment from the `comments` attribute can be obtained using `getDocComment()`.

Pretty printer
--------------

The pretty printer component compiles the AST back to PHP code. As the parser does not retain formatting
information the formatting is done using a specified scheme. Currently there is only one scheme available,
namely `PhpParser\PrettyPrinter\Standard`.

```php
<?php
$code = "<?php echo 'Hi ', hi\\getTarget();";

$parser        = new PhpParser\Parser(new PhpParser\Lexer);
$prettyPrinter = new PhpParser\PrettyPrinter\Standard;

try {
    // parse
    $stmts = $parser->parse($code);

    // change
    $stmts[0]         // the echo statement
          ->exprs     // sub expressions
          [0]         // the first of them (the string node)
          ->value     // it's value, i.e. 'Hi '
          = 'Hallo '; // change to 'Hallo '

    // pretty print
    $code = '<?php ' . $prettyPrinter->prettyPrint($stmts);

    echo $code;
} catch (PhpParser\Error $e) {
    echo 'Parse Error: ', $e->getMessage();
}
```

The above code will output:

    <?php echo 'Hallo ', hi\getTarget();

As you can see the source code was first parsed using `PhpParser\Parser->parse()`, then changed and then
again converted to code using `PhpParser\PrettyPrinter\Standard->prettyPrint()`.

The `prettyPrint()` method pretty prints a statements array. It is also possible to pretty print only a
single expression using `prettyPrintExpr()`.

Node traversation
-----------------

The above pretty printing example used the fact that the source code was known and thus it was easy to
write code that accesses a certain part of a node tree and changes it. Normally this is not the case.
Usually you want to change / analyze code in a generic way, where you don't know how the node tree is
going to look like.

For this purpose the parser provides a component for traversing and visiting the node tree. The basic
structure of a program using this `PhpParser\NodeTraverser` looks like this:

```php
<?php
$code = "<?php // some code";

$parser        = new PhpParser\Parser(new PhpParser\Lexer);
$traverser     = new PhpParser\NodeTraverser;
$prettyPrinter = new PhpParser\PrettyPrinter\Standard;

// add your visitor
$traverser->addVisitor(new MyNodeVisitor);

try {
    // parse
    $stmts = $parser->parse($code);

    // traverse
    $stmts = $traverser->traverse($stmts);

    // pretty print
    $code = '<?php ' . $prettyPrinter->prettyPrint($stmts);

    echo $code;
} catch (PhpParser\Error $e) {
    echo 'Parse Error: ', $e->getMessage();
}
```

A same node visitor for this code might look like this:

```php
<?php
class MyNodeVisitor extends PhpParser\NodeVisitorAbstract
{
    public function leaveNode(PhpParser\Node $node) {
        if ($node instanceof PhpParser\Node\Scalar\String) {
            $node->value = 'foo';
        }
    }
}
```

The above node visitor would change all string literals in the program to `'foo'`.

All visitors must implement the `PhpParser\NodeVisitor` interface, which defined the following four
methods:

    public function beforeTraverse(array $nodes);
    public function enterNode(PhpParser\Node $node);
    public function leaveNode(PhpParser\Node $node);
    public function afterTraverse(array $nodes);

The `beforeTraverse` method is called once before the traversal begins and is passed the nodes the
traverser was called with. This method can be used for resetting values before traversation or
preparing the tree for traversal.

The `afterTraverse` method is similar to the `beforeTraverse` method, with the only difference that
it is called once after the traversal.

The `enterNode` and `leaveNode` methods are called on every node, the former when it is entered,
i.e. before its subnodes are traversed, the latter when it is left.

All four methods can either return the changed node or not return at all (i.e. `null`) in which
case the current node is not changed. The `leaveNode` method can furthermore return two special
values: If `false` is returned the current node will be removed from the parent array. If an `array`
is returned the current node will be merged into the parent array at the offset of the current node.
I.e. if in `array(A, B, C)` the node `B` should be replaced with `array(X, Y, Z)` the result will be
`array(A, X, Y, Z, C)`.

Instead of manually implementing the `NodeVisitor` interface you can also extend the `NodeVisitorAbstract`
class, which will define empty default implementations for all the above methods.

The NameResolver node visitor
-----------------------------

One visitor is already bundled with the package: `PhpParser\NodeVisitor\NameResolver`. This visitor
helps you work with namespaced code by trying to resolve most names to fully qualified ones.

For example, consider the following code:

    use A as B;
    new B\C();

In order to know that `B\C` really is `A\C` you would need to track aliases and namespaces yourself.
The `NameResolver` takes care of that and resolves names as far as possible.

After running it most names will be fully qualified. The only names that will stay unqualified are
unqualified function and constant names. These are resolved at runtime and thus the visitor can't
know which function they are referring to. In most cases this is a non-issue as the global functions
are meant.

Also the `NameResolver` adds a `namespacedName` subnode to class, function and constant declarations
that contains the namespaced name instead of only the shortname that is available via `name`.

Example: Converting namespaced code to pseudo namespaces
--------------------------------------------------------

A small example to understand the concept: We want to convert namespaced code to pseudo namespaces
so it works on 5.2, i.e. names like `A\\B` should be converted to `A_B`. Note that such conversions
are fairly complicated if you take PHP's dynamic features into account, so our conversion will
assume that no dynamic features are used.

We start off with the following base code:

```php
<?php
const IN_DIR  = '/some/path';
const OUT_DIR = '/some/other/path';

// use the emulative lexer here, as we are running PHP 5.2 but want to parse PHP 5.3
$parser        = new PhpParser\Parser(new PhpParser\Lexer\Emulative);
$traverser     = new PhpParser\NodeTraverser;
$prettyPrinter = new PhpParser\PrettyPrinter\Standard;

$traverser->addVisitor(new PhpParser\NodeVisitor\NameResolver); // we will need resolved names
$traverser->addVisitor(new NodeVisitor\NamespaceConverter);     // our own node visitor

// iterate over all .php files in the directory
$files = new RecursiveIteratorIterator(new RecursiveDirectoryIterator(IN_DIR));
$files = new RegexIterator($files, '/\.php$/');

foreach ($files as $file) {
    try {
        // read the file that should be converted
        $code = file_get_contents($file);

        // parse
        $stmts = $parser->parse($code);

        // traverse
        $stmts = $traverser->traverse($stmts);

        // pretty print
        $code = '<?php ' . $prettyPrinter->prettyPrint($stmts);

        // write the converted file to the target directory
        file_put_contents(
            substr_replace($file->getPathname(), OUT_DIR, 0, strlen(IN_DIR)),
            $code
        );
    } catch (PhpParser\Error $e) {
        echo 'Parse Error: ', $e->getMessage();
    }
}
```

Now lets start with the main code, the `NodeVisitor_NamespaceConverter`. One thing it needs to do
is convert `A\\B` style names to `A_B` style ones.

```php
<?php
use PhpParser\Node;
class NodeVisitor_NamespaceConverter extends PhpParser\NodeVisitorAbstract
{
    public function leaveNode(Node $node) {
        if ($node instanceof Node\Name) {
            return new Node\Name($node->toString('_'));
        }
    }
}
```

The above code profits from the fact that the `NameResolver` already resolved all names as far as
possible, so we don't need to do that. All the need to create a string with the name parts separated
by underscores instead of backslashes. This is what `$node->toString('_')` does. (If you want to
create a name with backslashes either write `$node->toString()` or `(string) $node`.) Then we create
a new name from the string and return it. Returning a new node replaces the old node.

Another thing we need to do is change the class/function/const declarations. Currently they contain
only the shortname (i.e. the last part of the name), but they need to contain the complete class
name:

```php
<?php
use PhpParser\Node;
use PhpParser\Node\Stmt;
class NodeVisitor_NamespaceConverter extends PhpParser\NodeVisitorAbstract
{
    public function leaveNode(Node $node) {
        if ($node instanceof Node\Name) {
            return new Node\Name($node->toString('_'));
        } elseif ($node instanceof Stmt\Class_
                  || $node instanceof Stmt\Interface_
                  || $node instanceof Stmt\Function_) {
            $node->name = $node->namespacedName->toString('_');
        } elseif ($node instanceof Stmt\Const_) {
            foreach ($node->consts as $const) {
                $const->name = $const->namespacedName->toString('_');
            }
        }
    }
}
```

There is not much more to it than converting the namespaced name to string with `_` as separator.

The last thing we need to do is remove the `namespace` and `use` statements:

```php
<?php
use PhpParser\Node;
use PhpParser\Node\Stmt;
class NodeVisitor_NamespaceConverter extends PhpParser\NodeVisitorAbstract
{
    public function leaveNode(Node $node) {
        if ($node instanceof Node\Name) {
            return new Node\Name($node->toString('_'));
        } elseif ($node instanceof Stmt\Class_
                  || $node instanceof Stmt\Interface_
                  || $node instanceof Stmt\Function_) {
            $node->name = $node->namespacedName->toString('_');
        } elseif ($node instanceof Stmt\Const_) {
            foreach ($node->consts as $const) {
                $const->name = $const->namespacedName->toString('_');
            }
        } elseif ($node instanceof Stmt\Namespace_) {
            // returning an array merges is into the parent array
            return $node->stmts;
        } elseif ($node instanceof Stmt\Use_) {
            // returning false removed the node altogether
            return false;
        }
    }
}
```

That's all.