<p>I am creating a new table inside mysql and I am trying to add a foreign key constraint to one of the fields.</p>

<pre>
CREATE TABLE `onlineorder` (
  `receiptid` varchar(10) NOT NULL default '',
  `delivereddate` date default NULL,
  `cid` int(10) NOT NULL,
  `card#` int(10) default NULL,
  `expire` date default NULL,
  PRIMARY KEY  (`receiptid`),
  FOREIGN KEY (receiptid) REFERENCES purchase
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

</pre>

<p>However, after it creates it, I go into phpMyAdmin and export the table.  and it seems like the foreign key constraint has disappeared.</p>

<pre>
CREATE TABLE `onlineorder` (
  `receiptid` varchar(10) NOT NULL default '',
  `delivereddate` date default NULL,
  `cid` int(10) NOT NULL,
  `card#` int(10) default NULL,
  `expire` date default NULL,
  PRIMARY KEY  (`receiptid`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
</pre>

<p>Does phpMyAdmin get rid of foreign keys or am I doing something wrong here?</p>

You need to use the InnoDB engine to have foreign keys.
Ref:
http://dev.mysql.com/doc/refman/5.0/en/innodb-foreign-key-constraints.html