Source

File: wp-includes/post-template.php

function post_custom( $key = '' ) {
	$custom = get_post_custom();

	if ( ! isset( $custom[ $key ] ) ) {
		return false;
	} elseif ( 1 === count( $custom[ $key ] ) ) {
		return $custom[ $key ][0];
	} else {
		return $custom[ $key ];
	}
}