I know you are not after array_unique(). However, you will not find a magical obvious function nor will writing one be faster than making use of the native functions.

I propose:

function array_has_dupes($array) {   // streamline per @Felix   return count($array) !== count(array_unique($array));}

Adjust the second parameter of array_unique() to meet your comparison needs.

⚡ PERFORMANCE SOLUTION ⚡

If you care about performance and micro-optimizations check this one-liner:

function no_dupes(array $input_array) {    return count($input_array) === count(array_flip($input_array));}

Description:

Function compares number of array elements in $input_array with array_flip'ed elements. Values become keys and guess what - keys must be unique in associative arrays so not unique values are lost and final number of elements is lower than original.

As said in manual array keys can be only type of int or string so this is what you can have in original array values to compare, otherwise PHP will start casting with unexpected results.

PROOF FOR 10M RECORDS ARRAY

• Most voted solution: 14.187316179276s 🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌🐌
• Accepted solution: 2.0736091136932s 🐌🐌
• This answer solution: 0.14155888557434s 🐌/10

Test case:

<?php$elements = array_merge(range(1,10000000),[1]);$time = microtime(true);accepted_solution($elements);echo 'Accepted solution: ', (microtime(true) - $time), 's', PHP_EOL;$time = microtime(true);most_voted_solution($elements);echo 'Most voted solution: ', (microtime(true) - $time), 's', PHP_EOL;$time = microtime(true);this_answer_solution($elements);echo 'This answer solution: ', (microtime(true) - $time), 's', PHP_EOL;function accepted_solution($array){ $dupe_array = array(); foreach($array as $val){  // sorry, but I had to add below line to remove millions of notices  if(!isset($dupe_array[$val])){$dupe_array[$val]=0;}  if(++$dupe_array[$val] > 1){   return true;  } } return false;}function most_voted_solution($array) {   return count($array) !== count(array_unique($array));}function this_answer_solution(array $input_array) {    return count($input_array) === count(array_flip($input_array));}

Notice that accepted solution might be faster in certain condition when not unique values are near the beginning of huge array.

You can do:

function has_dupes($array) {    $dupe_array = array();    foreach ($array as $val) {        if (++$dupe_array[$val] > 1) {            return true;        }    }    return false;}