In case of slow and more time consuming query, several times I have found that, my php script timed out for the slower response of database server. And I could not get the data. Therefore I had nothing to process from the database.
But, If you use set_time_limit, in your php script and set a large time limit in case of slow and time consuming query, your php script will live to get the response and further processing.
The set_time_limit() function only affect the execution time of the script itself.
Example:
set_time_limit(200);
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 php.ini.
When called, set_time_limit() restarts the timeout counter from zero. 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.
From: php manual
