<?php
$arr = array(1=>'1233',2=>'12334',3 =>'Hello' ,4=>'HELLO', 5=>'U');
// Convert every value to uppercase, and remove duplicate values
$withoutDuplicates = array_unique(array_map("strtoupper", $arr));
// The difference in the original array, and the $withoutDuplicates array
// will be the duplicate values
$duplicates = array_diff($arr, $withoutDuplicates);
print_r($duplicates);