<?php
$string = <<<HTML
This is some sample data which is going to contain an image in the format <img src="http://www.randomdomain.com/randomfolder/randomimagename.jpg">. It will also contain lots of other text and maybe another image or two like this: <img alt='another image' src='http://www.example.com/randomfolder/randomimagename.jpg'>
HTML;
$srcs = [];
$dom=new DOMDocument;
$dom->loadHTML($string);
foreach ($dom->getElementsByTagName('img') as $img) {
$srcs[] = $img->getAttribute('src');
}
var_export($srcs);