How to add a foreign key in an existing table?

I have three tables:

1. User (id, name, birthdate) - PRIMARY KEY ('id')

2. Contact ( id, contact, verified ) - PRIMARY KEY ('id')

3. UserContact ( User_id, C开发者_JAVA技巧ontact_id ) - No Primary Key or Foriegn Key

Problem is that I did not care of primary key or foreign key while creating UserContact table. Therefore I am getting following error while inserting record in UserContact table in zend framework.

A table must have a primary key, but none was found

Now I want to create a MySql script that alter UserContact table and make User_id and Contact_id as foreign keys from User and Contact tables. How to write this script. I am using phpMyAdmin.

Thanks

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ALTER TABLE Syntax

Add foreign keys -

ALTER TABLE UserContact
  ADD CONSTRAINT FK_UserContact_User FOREIGN KEY (user_id) REFERENCES user(id);
ALTER TABLE UserContact
  ADD CONSTRAINT FK_UserContact_Contact FOREIGN KEY (contact_id) REFERENCES user(id);

Add primary key -

ALTER TABLE UserContact
  ADD PRIMARY KEY (User_id, Contact_id);