How to increment count in the replacement string when using preg_replace?

Use preg_replace_callback():

$count = 0;
preg_replace_callback('/test/', 'rep_count', $content);

function rep_count($matches) {
  global $count;
  return 'test' . $count++;
}

Use preg_replace_callback():

class TestReplace {
    protected $_count = 0;

    public function replace($pattern, $text) {
        $this->_count = 0;
        return preg_replace_callback($pattern, array($this, '_callback'), $text);
    }

    public function _callback($matches) {
        return 'test' . $this->_count++;
    }
}

$replacer = new TestReplace();
$replacer->replace('/test/', 'test test test'); // 'test0 test1 test2'

Note: Using global is the hard-and-fast solution but it introduces some problems, so I don't recommend it.