Get first image, if there's no image give the default Image URL

FirstImage.php

<?php
preg_match_all('~<img ([^>]+)>~i', $yourLongArticle, $matches);
$images = [];
foreach ($matches[1] as $str) {
    preg_match_all('~([a-z]([a-z0-9]*)?)=("|\')(.*?)("|\')~is', $str, $pairs);
    $images[] = array_combine($pairs[1], $pairs[4]);
}
if (!empty($images[0])) {
    return $images[0]['src'];
}
return "your default image URL";