replace everything except specific words in PHP

Is it possible to use php's preg_replace to remove anything in a string except specific words?

For example:

$text = 'Hello, this is a              test string from php.';

I want to remove everything except "test" and "php" so it will be:

$text will be 'test php'

You could always use a callback. Under PHP 5.3:

$keep = array('test'=>1, 'php'=>1);

$text = trim(
    preg_replace(
        '/[^A-Za-z]+/', ' ',
        preg_replace_callback(
            '/[A-Za-z]+/',
            function ($matched) use (&keep) {
                if (isset($keep[$matched[0]])) {
                    return $matched[0];
                }
                return '';
            }, $text
            )   )            );

Alternatively:

$text = 
    array_intersect(
        preg_split('/[^A-Za-z]+/', $text),
        array('test', 'php')
    );