I would suggest that you actually dig in and learn to do this rather than just try and implement code -- which if you are lucky enough to find -- will probably not be EXACTLY what you need. And you won't have the knowledge to modify if to suit your needs.
The PHP Manual is a great place to start. It has many examples in the functions area.
That aside, if I were you, I would use MySQL to store a couple of tables. One has your categories table:
CREATE TABLE `Categories` (
`CategoryID` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
`Category` VARCHAR( 100 ) NOT NULL ,
PRIMARY KEY ( `CategoryID` )
);
Store your categories here and learn how to query this table and echo each CategoryID and Category as an option in a select box via a loop.
Create another table to hold your bookmarks:
CREATE TABLE `Bookmarks` (
`BookmarkID` int(11) NOT NULL auto_increment,
`CategoryID` int(11) NOT NULL default '0',
`BookmarkURL` varchar(100) NOT NULL default '',
`BookmarkLabel` varchar(100) NOT NULL default '',
PRIMARY KEY (`BookmarkID`),
KEY `CategoryID` (`CategoryID`)
);
Have your form handler catch the categoryid from your dropdowns and insert the categoryid with the label and url in the Bookmarks table.
These new skills will help you facilitate displaying the values on your custom webpage.
Come back here frequently with solid questions in your process.
Finally, learn PHP and be happy ;-)