- array_combine: documentation ( source)
- array_merge: documentation ( source)
- range: documentation ( source)
<?php
$from = range(1, 25000);
$to = array_combine(range(25001, 50000), range(25001, 50000));
foreach($from as $i => $v) {
$to = array_merge($from, array($i => $v));
echo $i."\n";
}