<?php

// your code goes here

$array =[ 
  [
    'col_1' => 'one',
    'col_2' => 'two'
  ],
  [
    'col_5' => 'five',
    'col_3' => 'three'
  ]
];

$keys = array_map(
	function ($el) { 
		return array_search('three', $el) === false? -1 : 1; 
	}, $array);

print_r($keys);

$keys = array_map(
    function ($el) {
        $idx = array_search('three', $el);
        return $idx === false ? -1 : $idx; 
    }, $array);
    
    print_r($keys);
    
$keys = array_map(
    function ($el) {
        $idx = array_search('three', $el);
        if ($idx === false) {
        	return -1;
        } else {
        	return array_search($idx, array_keys($el));
        }
    }, $array);
    
        print_r($keys);
 $keys = array_map(
    function ($el) {
        return array_search('three', $el); 
    }, $array);
    
    print_r($keys);