In other words, if the timeout is the default 30 seconds, and 25 seconds into script execution a call such as set_time_limit( 20) is made, the script will run for a total of 45 seconds before timing out.
Set the number of seconds a script is allowed to run. If this is reached, the script returns a fatal error. The default limit is 30 seconds or, if it exists, the max_execution_time value defined in the configuration file. If seconds is set to zero, no time limit is imposed.