Timestamp (both PHP ones and MySQL's ones) are stored using 32 bits (i.e. 4 bytes) integers ; which means they are limited to a date range that goes from 1970 to 2038.
DATETIME don't have that limitation -- but are stored using more bytes (8 bytes, if I'm not mistaken)
After, between storing timestamps as seen by PHP, or timestamps as seen by MySQL :
- using PHP timestamps means manipulations are easier from PHP -- see Date/Time Functions
- using MySQL's timestamps means manipulations are easier from MySQL -- see 11.6. Date and Time Functions
And, for more informations between MySQL's TIMESTAMP and DATETIME datatypes, see 10.3.1. The DATETIME, DATE, and TIMESTAMP Types
