<?php
$txt = "lookSTARTfromhereSTARTagainhere";
$arr = explode("START", $txt); // Explode on START
unset($arr[0]); // first item is before START, we don't need that.
$res = [];
foreach($arr as $val){
$temp = str_split($val, 4); // Split array item on four letters.
$res = array_merge($res, $temp); // merge the new array with result array
}
var_dump($res);
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
array(5) {
[0]=>
string(4) "from"
[1]=>
string(4) "here"
[2]=>
string(4) "agai"
[3]=>
string(4) "nher"
[4]=>
string(1) "e"
}