PHP: how to use usort with anonymous function?

Question

I've an array of array.

I'm trying this code to sort main array based of a field of every single element of the main array.

$field = $this->sorting;
usort($this->out_table["rows"], function($a, $b) use ($field) {
        return strnatcmp($a[$field], $b[$field]);
});

But I got this

 Parse error: syntax error, unexpected T_FUNCTION 

Referred to the second line, the one which starts with 'usort'

What am I missing?

My php version is

PHP 5.2.4-2ubuntu5.27 with Suhosin-Patch 0.9.6.2 (cli) (built: Mar 11 2013 14:14:48)

Answer 1

PHP 5.2 doesn't support anonymous functions. Anonymous functions are instances of the Closure class, which as the docs say, wasn't introduced until 5.3... PS: _upgrade your PHP version, 5.2 was EOL'ed ages ago.

For now, though, you're best off writing your own class, pass the $field value to that class' instance and use the array-style callable argument:

class Sorter
{
    protected $field = null;
    public function __construct($field)
    {
        $this->field = $field;
    }
    public function sortCallback($a, $b)
    {
        return strnatcmp($a[$this->field], $b[$this->field]);
    }
}
$sorter = new Sorter($field);
usort($this->out_table["rows"], array($sorter, 'sortCallback'));

Which is basically what a Closure instance does, the anonymous function business is syntactic sugar in this case. The advantage of a class like this is that you could add some more sort-callbacks to it, and keep it handy as a sort of utility class with a sortAscending and sortDescending callback method, for example. Along with options you can set on the instance that make the sorter use strict (type and value) comparison where it's needed....