1. Set the email notification to an address you can check. Error messages can be helpful and you can always turn it off once you get the cron job working the way you want.
2. Use absolute paths in the script. The working directory seems to be /home/username , not the directory where the script is located.
Say your script is in /home/username/public_html
Say you want to create a file "cron_output.txt" to append to.
If you use fopen( "cron_output.txt" , "a" ) , the file will be in /home/username , not /home/username/public_html
You should use fopen( "/home/username/public_html/cron_output.txt" , "a" )