PHP Memory allocation issue:

[root@comp-server ~]# head -30000 mysql-slow.log > mysql-slow1.log
[root@comp-server ~]# php mysql_filter_slow_log.php mysql-slow1.log > mysql-slow-queries.log

The first 30,000 lines works ok. But it I select the first 40,000 then I get an error as displayed.

[root@comp-server ~]# head -40000 mysql-slow.log > mysql-slow1.log
[root@comp-server ~]# php mysql_filter_slow_log.php mysql-slow1.log > mysql-slow-queries.log
PHP Fatal error: Allowed memory size of 31457280 bytes exhausted (tried to allocate 71 bytes) in /root/mysql_filter_slow_log.php on line 545

How do I correct the problem and run the query on 40,000 lines instead of 30,000 lines?