How to access and manipulate multi-dimensional array by key names / path?

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Assuming $path is already an array via explode (or add to the function), then you can use references. You need to add in some error checking in case of invalid $path etc. (think isset):

$key = 'b.x.z';
$path = explode('.', $key);

Getter

function get($path, $array) {
    //$path = explode('.', $path); //if needed
    $temp =& $array;

    foreach($path as $key) {
        $temp =& $temp[$key];
    }
    return $temp;
}

$value = get($path, $arr); //returns NULL if the path doesn't exist

Setter / Creator

This combination will set a value in an existing array or create the array if you pass one that has not yet been defined. Make sure to define $array to be passed by reference &$array:

function set($path, &$array=array(), $value=null) {
    //$path = explode('.', $path); //if needed
    $temp =& $array;

    foreach($path as $key) {
        $temp =& $temp[$key];
    }
    $temp = $value;
}

set($path, $arr);
//or
set($path, $arr, 'some value');

Unsetter

This will unset the final key in the path:

function unsetter($path, &$array) {
    //$path = explode('.', $path); //if needed
    $temp =& $array;

    foreach($path as $key) {
        if(!is_array($temp[$key])) {
            unset($temp[$key]);
        } else {
            $temp =& $temp[$key];
        }
    }
}
unsetter($path, $arr);

*The original answer had some limited functions that I will leave in case they are of use to someone:

Setter

Make sure to define $array to be passed by reference &$array:

function set(&$array, $path, $value) {
    //$path = explode('.', $path); //if needed
    $temp =& $array;

    foreach($path as $key) {
        $temp =& $temp[$key];
    }
    $temp = $value;
}

set($arr, $path, 'some value');

Or if you want to return the updated array (because I’m bored):

function set($array, $path, $value) {
    //$path = explode('.', $path); //if needed
    $temp =& $array;

    foreach($path as $key) {
        $temp =& $temp[$key];
    }
    $temp = $value;

    return $array;
}

$arr = set($arr, $path, 'some value');

Creator

If you wan’t to create the array and optionally set the value:

function create($path, $value=null) {
    //$path = explode('.', $path); //if needed
    foreach(array_reverse($path) as $key) {
        $value = array($key => $value);
    }
    return $value;
}    

$arr = create($path);    
//or
$arr = create($path, 'some value');

For Fun

Constructs and evaluates something like $array['b']['x']['z'] given a string b.x.z:

function get($array, $path) {
    //$path = explode('.', $path); //if needed
    $path = "['" . implode("']['", $path) . "']";
    eval("\$result = \$array{$path};");

    return $result;
}

Sets something like $array['b']['x']['z'] = 'some value';:

function set(&$array, $path, $value) {
    //$path = explode('.', $path); //if needed
    $path = "['" . implode("']['", $path) . "']";
    eval("\$array{$path} = $value;");
}

Unsets something like $array['b']['x']['z']:

function unsetter(&$array, $path) {
    //$path = explode('.', $path); //if needed
    $path = "['" . implode("']['", $path) . "']";
    eval("unset(\$array{$path});");
}

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