<?php
$xml = '<set1>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set1>
<set2>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set2>';
$xml = preg_replace('/<uid1>([\n])<\/uid1>/', 'dee', $xml);
//preg_match('/<uid1>#([\n])<\/uid1>/', $file, $match);
echo $xml;
<set1>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set1>
<set2>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set2>
Output for 8.3.5
Warning: PHP Startup: Unable to load dynamic library 'sodium.so' (tried: /usr/lib/php/8.3.5/modules/sodium.so (libsodium.so.23: cannot open shared object file: No such file or directory), /usr/lib/php/8.3.5/modules/sodium.so.so (/usr/lib/php/8.3.5/modules/sodium.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
<set1>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set1>
<set2>
<uid1>Line 1
Line 2
Line 3
Line 4
</uid1>
<uid20>Line 1
Line 2
Line 3
Line 4
</uid20>
</set2>