function uniqerand($len = 7) {
    $word = array_merge(range('a', 'z'), range('A', 'Z'), range('0', '9'));
    shuffle($word);
    return substr(implode($word), 0, $len);
}

echo 'ABC'.uniqerand();

function randomString() {
	var chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz";
	var string_length = 7;
	var randomstring = '';
	for (var i=0; i<string_length; i++) {
		var rnum = Math.floor(Math.random() * chars.length);
		randomstring += chars.substring(rnum,rnum+1);
	}
	return 'ABC'+randomstring;
}

alert(randomString());