Understanding the abstract syntax tree

As a developer, it might be of interest for you to be free from certain syntax restrictions imposed in PHP 5 and earlier. Aside from the uniformity of the syntax mentioned previously, where you'll see the most improvement in syntax is the ability to call any return value, which is callable by simply appending an extra set of parentheses. Also, you'll be able to directly access any array element when the return value is an array.

How to do it...

Any function or method that returns a callback can be immediately executed by simply appending parentheses () (with or without parameters). An element can be immediately dereferenced from any function or method that returns an array by simply indicating the element using square brackets [];. In the short (but trivial) example shown next, the function test() returns an array. The array contains six anonymous functions. $a has a value of $t. $$a is interpreted as $test:
```
function test()
{
    return [
        1 => function () { return [
            1 => function ($a) { return 'Level 1/1:' . ++$a; },
            2 => function ($a) { return 'Level 1/2:' . ++$a; },
        ];},
        2 => function () { return [
            1 => function ($a) { return 'Level 2/1:' . ++$a; },
            2 => function ($a) { return 'Level 2/2:' . ++$a; },
        ];}
    ];
}

$a = 't';
$t = 'test';
echo $$a()[1]()[2](100);
```
AST allows us to issue the echo $$a()[1]()[2](100) command. This is parsed left-to-right, which executes as follows:
- $$a() interprets as test(), which returns an array
- [1] dereferences array element 1, which returns a callback
- () executes this callback, which returns an array of two elements
- [2] dereferences array element 2, which returns a callback
- (100) executes this callback, supplying a value of 100, which returns Level 1/2:101
  Tip
  Such a statement is not possible in PHP 5: a parse error would be returned.

The following is a more substantive example that takes advantage of AST syntax to define a data filtering and validating class. First of all, we define the Application\Web\Securityclass. In the constructor, we build and define two arrays. The first array consists of filter callbacks. The second array has validation callbacks:

public function __construct()
  {
    $this->filter = [
      'striptags' => function ($a) { return strip_tags($a); },
      'digits'    => function ($a) { return preg_replace(
      '/[^0-9]/', '', $a); },
      'alpha'     => function ($a) { return preg_replace(
      '/[^A-Z]/i', '', $a); }
    ];
    $this->validate = [
      'alnum'  => function ($a) { return ctype_alnum($a); },
      'digits' => function ($a) { return ctype_digit($a); },
      'alpha'  => function ($a) { return ctype_alpha($a); }
    ];
  }

We want to be able to call this functionality in a developer-friendly manner. Thus, if we want to filter digits, then it would be ideal to run a command such as this:
```
$security->filterDigits($item));
```

To accomplish this we define the magic method __call(), which gives us access to non-existent methods:

public function __call($method, $params)
{

  preg_match('/^(filter|validate)(.*?)$/i', $method, $matches);
  $prefix   = $matches[1] ?? '';
  $function = strtolower($matches[2] ?? '');
  if ($prefix && $function) {
    return $this->$prefix[$function]($params[0]);
  }
  return $value;
}

We use preg_match() to match the $method param against filter or validate. The second sub-match will then be converted into an array key in either $this->filter or $this->validate. If both sub-patterns produce a sub-match, we assign the first sub-match to $prefix, and the second sub-match $function. These end up as variable parameters when executing the appropriate callback.

Tip

Don't go too crazy with this stuff!

As you revel in your new found freedom of expression, made possible by AST, be sure to keep in mind that the code you end up writing could, in the long run, be extremely cryptic. This will ultimately cause long-term maintenance problems.

How it works...

First of all, we create a sample file, chap_02_web_filtering_ast_example.php, to take advantage of the autoloading class defined in Chapter 1, Building the Foundation, to obtain an instance of Application\Web\Security:

require __DIR__ . '/../Application/Autoload/Loader.php';
Application\Autoload\Loader::init(__DIR__ . '/..');
$security = new Application\Web\Security();

Next, we define a block of test data:

$data = [
    '<ul><li>Lots</li><li>of</li><li>Tags</li></ul>',
    12345,
    'This is a string',
    'String with number 12345',
];

Finally, we call each filter and validator for each item of test data:

foreach ($data as $item) {
  echo 'ORIGINAL: ' . $item . PHP_EOL;
  echo 'FILTERING' . PHP_EOL;
  printf('%12s : %s' . PHP_EOL,'Strip Tags', $security->filterStripTags($item));
  printf('%12s : %s' . PHP_EOL, 'Digits', $security->filterDigits($item));
  printf('%12s : %s' . PHP_EOL, 'Alpha', $security->filterAlpha($item));
    
  echo 'VALIDATORS' . PHP_EOL;
  printf('%12s : %s' . PHP_EOL, 'Alnum',  
  ($security->validateAlnum($item))  ? 'T' : 'F');
  printf('%12s : %s' . PHP_EOL, 'Digits', 
  ($security->validateDigits($item)) ? 'T' : 'F');
  printf('%12s : %s' . PHP_EOL, 'Alpha',  
  ($security->validateAlpha($item))  ? 'T' : 'F');
}

Here is the output of some input strings:

Table of Contents for
PHP 7: Real World Application Development

Understanding the abstract syntax tree

How to do it...

Tip

Tip

How it works...

See also

Table of Contents for PHP 7: Real World Application Development

Understanding the abstract syntax tree

How to do it...

Tip

Tip

How it works...

See also

Table of Contents for
PHP 7: Real World Application Development