What is the ideal data type to use when storing latitude / longitude in a MySQL database?

Use MySQL's spatial extensions with GIS.

Google provides a start to finish PHP/MySQL solution for an example "Store Locator" application with Google Maps. In this example, they store the lat/lng values as "Float" with a length of "10,6"

http://code.google.com/apis/maps/articles/phpsqlsearch.html

Basically it depends on the precision you need for your locations. Using DOUBLE you'll have a 3.5nm precision. DECIMAL(8,6)/(9,6) goes down to 16cm. FLOAT is 1.7m...

This very interesting table has a more complete list: http://mysql.rjweb.org/doc.php/latlng :

Datatype               Bytes            Resolution

Deg*100 (SMALLINT)     4      1570 m    1.0 mi  Cities
DECIMAL(4,2)/(5,2)     5      1570 m    1.0 mi  Cities
SMALLINT scaled        4       682 m    0.4 mi  Cities
Deg*10000 (MEDIUMINT)  6        16 m     52 ft  Houses/Businesses
DECIMAL(6,4)/(7,4)     7        16 m     52 ft  Houses/Businesses
MEDIUMINT scaled       6       2.7 m    8.8 ft
FLOAT                  8       1.7 m    5.6 ft
DECIMAL(8,6)/(9,6)     9        16cm    1/2 ft  Friends in a mall
Deg*10000000 (INT)     8        16mm    5/8 in  Marbles
DOUBLE                16       3.5nm     ...    Fleas on a dog

Hope this helps.