Find a value in an array by key recursively

findByKey.php

<?php

// Note: 
//   Whether $list contains a NULL value for $needle at some level, or there is no $needle at all,
//   in both cases this function returns NULL.
//   If you want to find non-NULL $needle only, replace `array_key_exists` with `isset`

function findByKey($list, $needle) {
    $stack = [$list];
    while( !empty($stack) ) {
        $list = array_shift($stack);
        if ( array_key_exists($needle, $list) ) return $list[$needle];
        foreach($list as $k => $l) if ( is_array($l) ) $stack[] = $l;
    } 
}

// This version is faster, but traversation order is changes (deepest first)
function findByKeyR($list, $needle) {
    $stack = [$list];
    while( !empty($stack) ) {
        $list = array_pop($stack);
        if ( array_key_exists($needle, $list) ) return $list[$needle];
        foreach($list as $k => $l) if ( is_array($l) ) $stack[] = $l;
    } 
}

findByKeyRec.php

<?php

// Recursive version of the above
function findByKeyRec($list, $needle, &$found=NULL) {
    if ( $found = array_key_exists($needle, $list) ) return $list[$needle];
    foreach($list as $k => $l) if ( is_array($l) ) {
      $r = findByKeyRec($l, $needle, $found);
      if ( $found ) return $r;
    }  
}