$string = 'I wish you a merry <span style="font-size: 14px;">Christmas</span> and a happy new <span style="font-size: 18px;">year</span>!'

$arr[0] = 'I wish you a merry '
$arr[1] = '<span style="font-size: 14px;">Christmas</span>'
$arr[2] = ' and a happy new '
$arr[3] = '<span style="font-size: 18px;">year</span>'
$arr[4] = '!'

$arr = explode('<span style="font-size: ', $string);

preg_match_all('~<span style="font-size:' . $fontSize . 'px;">(.*?)</span>~', $string, $output[$fontSize]);