Cambiar texto a lowercase usando expresiones regulares.

[NSD]

Código PHP:

<?php 
$txt = 'JXpHzkCrdoXD6R59gHqP3%2Fdz8bDzB8aNpze0w6QCE7Q%2F16jtZ%2F1fx7zHmtQG1UVv%2BI4NDVnygp7FhCfVSAPBsQ%3D%3D';
    $partes = array_filter(array_map(function($ele){ return strtolower(substr($ele, 0, 2)); }, explode("%", $txt)), "ctype_xdigit");
    var_dump($partes);
 

Salida:

array (size=6)
1 => string '2f' (length=2)
2 => string '2f' (length=2)
3 => string '2f' (length=2)
4 => string '2b' (length=2)
5 => string '3d' (length=2)
6 => string '3d' (length=2)