$string='$5.63';  //or for the french $string='5,63$';

$patterns = array();
$patterns[0] = '/\$/';
$patterns[1] = '/,/';
$replacements = array();
$replacements[2] = '';
$replacements[1] = '.';

echo preg_replace($patterns, $replacements, $string);

//result=5.63