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

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});");
}

I have solution for you not in the pure PHP, but using ouzo goodies concretely Arrays::getNestedValue method:

$arr = array('a' => 1,
    'b' => array(
        'y' => 2,
        'x' => array('z' => 5, 'w' => 'abc')
    ),
    'c' => null);

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

print_r(Arrays::getNestedValue($arr, $path));

Similarly if you need to set nested value you can use Arrays::setNestedValue method.

$arr = array('a' => 1,
    'b' => array(
        'y' => 2,
        'x' => array('z' => 5, 'w' => 'abc')
    ),
    'c' => null);

Arrays::setNestedValue($arr, array('d', 'e', 'f'), 'value');
print_r($arr);

I have a utility I regularly use that I'll share. The difference being it uses array access notation (e.g. b[x][z]) instead of dot notation (e.g. b.x.z). With the documentation and code it is fairly self-explanatory.

<?php
class Utils {
    /**
     * Gets the value from input based on path.
     * Handles objects, arrays and scalars. Nesting can be mixed.
     * E.g.: $input->a->b->c = 'val' or $input['a']['b']['c'] = 'val' will
     * return "val" with path "a[b][c]".
     * @see Utils::arrayParsePath
     * @param mixed $input
     * @param string $path
     * @param mixed $default Optional default value to return on failure (null)
     * @return NULL|mixed NULL on failure, or the value on success (which may also be NULL)
     */
    public static function getValueByPath($input,$path,$default=null) {
        if ( !(isset($input) && (static::isIterable($input) || is_scalar($input))) ) {
            return $default; // null already or we can't deal with this, return early
        }
        $pathArray = static::arrayParsePath($path);
        $last = &$input;
        foreach ( $pathArray as $key ) {
            if ( is_object($last) && property_exists($last,$key) ) {
                $last = &$last->$key;
            } else if ( (is_scalar($last) || is_array($last)) && isset($last[$key]) ) {
                $last = &$last[$key];
            } else {
                return $default;
            }
        }
        return $last;
    }

    /**
     * Parses an array path like a[b][c] into a lookup array like array('a','b','c')
     * @param string $path
     * @return array
     */
    public static function arrayParsePath($path) {
        preg_match_all('/\\[([^[]*)]/',$path,$matches);
        if ( isset($matches[1]) ) {
            $matches = $matches[1];
        } else {
            $matches = array();
        }
        preg_match('/^([^[]+)/',$path,$name);
        if ( isset($name[1]) ) {
            array_unshift($matches,$name[1]);
        } else {
            $matches = array();
        }
        return $matches;
    }

    /**
     * Check if a value/object/something is iterable/traversable, 
     * e.g. can it be run through a foreach? 
     * Tests for a scalar array (is_array), an instance of Traversable, and 
     * and instance of stdClass
     * @param mixed $value
     * @return boolean
     */
    public static function isIterable($value) {
        return is_array($value) || $value instanceof Traversable || $value instanceof stdClass;
    }
}

$arr = array('a' => 1,
             'b' => array(
                 'y' => 2,
                 'x' => array('z' => 5, 'w' => 'abc')
             ),
             'c' => null);

$key = 'b[x][z]';

var_dump(Utils::getValueByPath($arr,$key)); // int 5

?>

As a "getter", I've used this in the past:

$array = array('data' => array('one' => 'first', 'two' => 'second'));

$key = 'data.one';

function find($key, $array) {
    $parts = explode('.', $key);
    foreach ($parts as $part) {
        $array = $array[$part];
    }
    return $array;
}

$result = find($key, $array);
var_dump($result);