- var_dump: documentation ( source)
- preg_match_all: documentation ( source)
<?php
$raw_string = '152124687951<?xml version="1.0"><culo>Amazing</culo></Document>65464614
abc<?xml version="1.0"><culo>Amazing</culo></Document>abc 152124687951<?xml version="1.0"><culo>Amazing</culo></Document>65464614
abc<?xml version="1.0"><culo>Amazing</culo></Document>abc';
$pattern = '@\\<\\?xml.+Document\\>@';
preg_match_all($pattern, $raw_string, $matches, PREG_PATTERN_ORDER);
var_dump($matches);