$orig_array = array(
	'fg_foo' => 'blah',
	'fg_bar' => 'blah',
);

$new_array = array(
	'foo' => 'blah',
	'bar' => 'blah',
);

$keys = array_keys($orig_array);
$values = array_values($orig_array);
$data = array();

foreach ($keys as $count => $keyname) {
	if ('fg_' == substr($keyname, 0, 3)) {
		$data[substr($keyname, 3)] = $values[$count];
	}
}