As commented, you can do this only with the functions array_map and explode, still with the array_filter to remove possible unwanted results:
function relacao_tipo_metragem($tipo, $metragem) {
    if ($tipo and $metragem) {
        return "{$tipo}: {$metragem}";
    }
    return null;
}
$tipo = explode(',', 'a,b,c,d');
$metragem = explode(',', '15,18,32,44');
$dados = array_filter(array_map('relacao_tipo_metragem', $tipo, $metragem));
print_r($dados);
Generating the result:
Array
(
    [0] => a: 15
    [1] => b: 18
    [2] => c: 32
    [3] => d: 44
)
If one of the values, either type or length, is not set, the column will be ignored.
							
							
						 
You can use the
explode. Documentation and work on it. How it will generatearrays, is interesting this topic: Merge all values at the same array level, at the same level– rbz