<?php
$str = "this is a Test String to get the last index of word with an uppercase letter in PHP";
$pattern = "/.*\s([A-Z])(\w+)/";
//$pattern = "/.*\s([A-Z])[A-Z]+/";
preg_match($pattern, $str, $match);
$letter = $match[1];
$word = $match[1] . $match[2];
$position = strrpos($str, $match[1].$match[2]);
echo "Letter to find: " . $letter . "\nWord to find: " . $word . "\nPosition of letter: " . $position;
Letter to find: P
Word to find: PHP
Position of letter: 80
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
Letter to find: P
Word to find: PHP
Position of letter: 80