array_key_exists for multidimensional array

Hi!

Bellow an example of usage array_key_exists for one dimensional arrat
(MyArr1). How I call array_key_exists for multidimensional (MyArr2) ?

Thanks

<?php
$MyArr1=array();
$MyArr1['A']=1;
if (array_key_exists('A', $MyArr1))
print "A exists";
else
print "A doesnt exist";


$MyArr2=array();
$MyArr2['A'][1] = 1;
$MyArr2['A'][2] = 1;

if (array_key_exists(????????????))
print "A1 exists";
else
print "A1 doesnt exist";
?>