PHP Manual: Registers a PHP function for use as an SQL aggregate function

public bool SQLite3::createAggregate ( string $name , mixed $step_callback , mixed $final_callback [, int $argument_count = -1 ] )

Registers a PHP function or user-defined function for use as an SQL aggregate function for use within SQL statements.

name: Name of the SQL aggregate to be created or redefined.
step_callback: The name of a PHP function or user-defined function to apply as a callback for every item in the aggregate.
final_callback: The name of a PHP function or user-defined function to apply as a callback at the end of the aggregate data.
argument_count: The number of arguments that the SQL aggregate takes. If this parameter is negative, then the SQL aggregate may take any number of arguments.

Returns TRUE upon successful creation of the aggregate, FALSE on failure.