toptalTestBetterSolution.php

<?php
function test($v, $arr1) {
	$arr2 = [];
	while(1){
		$count1 = isset(array_count_values($arr1)[$v]) ? array_count_values($arr1)[$v] : 0;
		$count2 = isset(array_count_values($arr2)[$v]) ? array_count_values($arr2)[$v] : 0;
		if(count($arr1) == 0 || $count1 == count($arr2) - $count2) {
			return count($arr1);
		}
		$arr2[] = array_shift($arr1);
	}
}
echo test(5, [5,5,1,7,2,3,5]);

i`ve done a better solution in relation of performance.