Pass PHP() code to evaluate:
mysql> SELECT PHP("return strftime('%c');");
+-------------------------------+
| PHP("return strftime('%c');") |
+-------------------------------+
| Fri May  9 13:52:49 2003      |
+-------------------------------+
Arguments are available in $argv:
mysql> SELECT PHP('return $argv[1] * $argv[2];',4,5);
+----------------------------------------+
| PHP('return $argv[1] * $argv[2];',4,5) |
+----------------------------------------+
| 20                                     |
+----------------------------------------+