An output of the entire inheritance chain using closures, recursion, and OOP

class ParentClass {
    public static function getChain() {
        $chain = null;
        return $function = function($className='') use (& $chain, & $function) {
            if (empty($className))
                $className = static::class;

            if (empty($chain))
                $chain = $className;

            $parent = get_parent_class($className);

            if ($parent !== false) {
                $chain .= " > {$parent}";
                return $function($parent);
            }

            return $chain;
        };
    }
}

class Child extends ParentClass {}
class SubChild extends Child {}
class Sub2 extends SubChild {}
class Sub3 extends Sub2 {}
class Sub4 extends Sub3 {}
class Sub5 extends Sub4 {}
class Sub6 extends Sub5 {}
class Sub7 extends Sub6 {}

printf("%s\n", Sub7::getChain()());

$getChain = Sub7::getChain();
printf("%s\n", $getChain('Sub3'));

Output is:
Sub7 > Sub6 > Sub5 > Sub4 > Sub3 > Sub2 > SubChild > Child > ParentClass
Sub3 > Sub2 > SubChild > Child > ParentClass

Note that from PHP 5.5 you can also use `parent::class` from within a method, e.g.

<?php
    function child()
    {
        echo "I'm ", parent::class, "'s son\n";
    }
?>

Looks a bit tidier and technically probably more optimal, as it avoids a function call lookup.

You can use this function to find common parent of multiple objects or classes.

<?php
/**
 * Returns name of the first (in class hierarchy) common parent class of all provided objects or classes.
 * Returns FALSE when common class is not found.
 *
 * @param mixed $objects Array that can contain objects or class names.
 * @return mixed
 */
function get_first_common_parent($objects) {
    $common_ancestors = null;
    foreach($objects as $object) {
        if (is_object($object)) {
            $class_name = get_class($object);
        } else {
            $class_name = $object;
        }
        
        $parent_class_names = array();
        $parent_class_name = $class_name;
        do {
            $parent_class_names[] = $parent_class_name;
        } while($parent_class_name = get_parent_class($parent_class_name));
        
        if ($common_ancestors === null) {
            $common_ancestors = $parent_class_names;
        } else {
            $common_ancestors = array_intersect($common_ancestors, $parent_class_names);
        }
    }
    
    return reset($common_ancestors);
}
?>

Example:

<?php
class A {
}

    class B extends A {
    }
    
        class D extends B {
        }
        
        class E extends B {
        }

    class C extends A {
    }

        class F extends C {
        }
    
            class G extends F {
            }

class H {
}

//returns "A"
get_first_common_parent(array('G', 'E'));

//returns "F"
get_first_common_parent(array(new G(), 'F'));

//returns false (no common parent)
get_first_common_parent(array('C', 'H'));

//returns false (non-existent class provided)
get_first_common_parent(array(new B(), 'X'));
?>

I wrote a simple function doing the reverse thing: get the children:

<?php
function get_child($instance, $classname) {
    $class = $classname;
    $t = get_class($instance);
    while (($p = get_parent_class($t)) !== false) {
        if ($p == $class) {
            return $t;
        }
        $t = $p;
    }
    return false;
}

abstract class A {
    function someFunction() {
        return get_child($this, __CLASS__);
    }
}

class B extends A {

}

class C extends B {

}

$c = new C();
echo $c->someFunction(); //displays B

?>

"'If called without parameter outside object' What on earth does that mean?"

There are two places this could be called:
1. From within a member function of an object.  In this case, it may be called with no parameters and will return the parent class of the object owning the member function.  (If the parameter is included, then it will return the parent class of the specified class as normal.)

2. From outside an object (i.e., global or function scope).  In this case, PHP doesn't know what class you're talking about if you don't include a parameter, so it returns FALSE.  (But, of course, it works if you specify the class with the parameter.)

PHP (4 at least, dunno about 5) stores classnames in lower case, so:

<?PHP

class Foo
{
}

class Bar extends Foo
{
}

echo get_parent_class('Bar');

echo "\n";

echo get_parent_class('bar');

?>

will output:

foo
foo

If the argument obj is a string and the class is not defined, then the function returns FALSE.

If the argument obj is an object created from a class with no ancestors (or a string representing a class with no ancestors), then the function returns FALSE.