The PHP `date()' function expects a number for the second parameter - ie a unix timestamp.
You can convert a SQL date string (or virtually any other date string) into a timestamp in PHP by using the strtotime() function. At least two other answers have already suggested this.
However, I would suggest that you'd be better off getting the date out of your database in unix timestamp format in the first place. You can do this by querying using the MySQL UNIX_TIMESTAMP() function, as follows:
SELECT UNIX_TIMESTAMP(mydatefield) AS mydatefield_timestamp FROM mytable
..obviously, replacing the field and table names as appropriate.
Then you will get the date in timestamp format in your returned dataset in PHP, which you can pass directly into the date() function as follows:
echo date('F/j/Y',$result['mydatefield_timestamp']);
Hope that helps.