Understanding differences in parsing

In PHP 5, expressions on the right side of an assignment operation were parsed right-to-left. In PHP 7, parsing is consistently left-to-right.

How to do it...

A variable-variable is a way of indirectly referencing a value. In the following example, first $$foo is interpreted as ${$bar}. The final return value is thus the value of $bar instead of the direct value of $foo (which would be bar):
```
$foo = 'bar';
$bar = 'baz';
echo $$foo; // returns  'baz'; 
```
In the next example we have a variable-variable $$foo, which references a multi-dimensional array with a bar key and a baz sub-key:
```
$foo = 'bar';
$bar = ['bar' => ['baz' => 'bat']];
// returns 'bat'
echo $$foo['bar']['baz'];
```
In PHP 5, parsing occurs right-to-left, which means the PHP engine would be looking for an $foo array, with a bar key and a baz. sub-key The return value of the element would then be interpreted to obtain the final value ${$foo['bar']['baz']}.
In PHP 7, however, parsing is consistently left-to-right, which means that $foo is interpreted first ($$foo)['bar']['baz'].
In the next example you can see that $foo->$bar['bada'] is interpreted quite differently in PHP 5, compared with PHP 7. In the following example, PHP 5 would first interpret $bar['bada'], and reference this return value against a $foo object instance. In PHP 7, on the other hand, parsing is consistently left-to-right, which means that $foo->$bar is interpreted first, and expects an array with a bada element. You will also note, incidentally, that this example uses the PHP 7 anonymous class feature:
```
// PHP 5: $foo->{$bar['bada']}
// PHP 7: ($foo->$bar)['bada']
$bar = 'baz';
// $foo = new class 
{ 
    public $baz = ['bada' => 'boom']; 
};
// returns 'boom'
echo $foo->$bar['bada'];
```

The last example is the same as the one immediately above, except that the return value is expected to be a callback, which is then immediately executed as follows:

// PHP 5: $foo->{$bar['bada']}()
// PHP 7: ($foo->$bar)['bada']()
$bar = 'baz';
// NOTE: this example uses the new PHP 7 anonymous class feature
$foo = new class 
{ 
     public function __construct() 
    { 
        $this->baz = ['bada' => function () { return 'boom'; }]; 
    } 
};
// returns 'boom'
echo $foo->$bar['bada']();

How it works...

Place the code examples illustrated in 1 and 2 into a single PHP file that you can call chap_02_understanding_diffs_in_parsing.php. Execute the script first using PHP 5, and you will notice that a series of errors will result, as follows:

The reason for the errors is that PHP 5 parses inconsistently, and arrives at the wrong conclusion regarding the state of the variable variables requested (as previously mentioned). Now you can go ahead and add the remaining examples, as shown in steps 5 and 6. If you then run this script in PHP 7, the results described will appear, as shown here:

Table of Contents for
PHP 7: Real World Application Development

Understanding differences in parsing

How to do it...

How it works...

See also

Table of Contents for PHP 7: Real World Application Development

Understanding differences in parsing

How to do it...

How it works...

See also

Table of Contents for
PHP 7: Real World Application Development