How can we increase the execution time of a php script?

By the use of void set_time_limit(int seconds)

Set number of seconds a script is allowed to run. If this is reached, script returns a fatal error. Default limit is 30 seconds or, if it exists, max_execution_time value defined in the php.ini. If seconds is set to zero, no time limit is imposed.

When called, set_time_limit() restarts timeout counter from zero. Or we can say, if timeout is the default 30 seconds, and 25 seconds into script execution a call like set_time_limit(20) is made, script will run for a total of 45 seconds before timing out.