<?php
 
// your code goes here
 
$arr[] = [
            'what' => 'b4',
            'map' => '74,76,77,83'
        ];
$arr[] = [
            'what' => 'b2',
            'map' => '53,82'
        ];
$arr[] = [
	        'what' => 'b1',
            'map' => '36'
        ];
 
echo abc('b4', $arr);
 
function abc($what, $arr){
	$map = null;
	$idx = array_search($what, array_column($arr, 'what'));
	if ($idx !== false) {
		$map = $arr[$idx]['map'];
	}
    return $map;
}

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