From SA-MP Wiki
This page contains an overview of all the callbacks available in SA-MP. Callbacks are called in the script by the server when something important happens, something closely related to the name of the callback (e.g. OnPlayerSpawn is called when a player spawns). Most callbacks have playerid as a parameter to indicate the player for whom the event occurred however not all do (as some aren't relevant to a player).
重要: When using these callbacks in a filterscript, make sure you return 1, so it's available in other filterscripts too. When you don't return a value or return 0, other callbacks in filterscripts loaded at a later time aren't executed.
OnPlayerCommandText uses the opposite: 1 is indicating "command found" to all scripts, 0 will show "Unknown Command".