Array
(
    [0] => Array
        (
            [L] => 2
            [M] => a
        )

    [1] => Array
        (
            [L] => 1
            [M] => b
        )

    [2] => Array
        (
            [L] => 0
            [M] => c
        )

)

<?php

$array = array(array('L'=>2, 'M'=>'a'),array('L'=>1, 'M'=>'b'),array('L'=>0, 'M'=>'c'));

$i=0;
$x='b'; // пусть x == b

function tryAgain() {

global $x, $i, $array;

if (isset($array[$i]['M'])) {
if ($x==$array[$i]['M']) {
echo "Совпадение - \$x == ".$array[$i]['M'];
exit;
}
else {
++$i; tryAgain();
}
}
else {echo "Совпадений не найдено!"; exit;}
}

tryAgain();

?>

$arr = Array
(
    Array ('L' => 2, 'M' => 'a'),
    Array ('L' => 1, 'M' => 'b'),
    Array ('L' => 0, 'M' => 'c')
);

function hasX($var) {
  $x = 'b';
  return ($var['M'] == $x);
}

print_r(array_filter($arr, "hasX"));

<?php
$arr = Array
(
    Array ('L' => 2, 'M' => 'a'),
    Array ('L' => 1, 'M' => 'b'),
    Array ('L' => 0, 'M' => 'c')
);

$x = 'b';

function hasX($var) {
  $x = 'b';
  return ($var['M'] == $x);
}

$start = microtime(true);
for ($i =0; $i < 10000; $i++) {
  array_filter($arr, "hasX");
}
$end = microtime(true);

echo 'array_filter:', ($end - $start) , '<br>';

$start = microtime(true);
for ($i =0; $i < 10000; $i++) {
  foreach ($arr as $key => $val) {
    if ($val['M'] == $x) {
      // echo $key
      $key;
    }
  }
}
$end = microtime(true);

echo 'тупо перебор: ', ($end - $start) , '<br>';

array_filter:0.178192853928
тупо перебор: 0.0122048854828

$arr = Array
(
    Array ('L' => 2, 'M' => 'a'),
    Array ('L' => 1, 'M' => 'b'),
    Array ('L' => 0, 'M' => 'c')
);

function hasX($var) {
  $x = 'b';
  return ($var['M'] == $x);
}

print_r(array_filter($arr, "hasX"));