- unserialize: documentation ( source)
<?php
$img = 'a:1:{i:0;a:3:{s:8:"fullpath";s:76:"https://www.educationfolder.com/files1/user-pics/4/142944540636159131_ml.jpg";s:5:"image";s:27:"4/142944540636159131_ml.jpg";s:10:"mention_id";s:3:"398";}}';
$img = unserialize($img);
echo $img[0]['fullpath'];