This code will create a cron job for executing "yourfile.php" frequently
First create a text file name "cronstat.txt" with following content
30 lynx -dump http://yoursite/yourfile.php
Now create another file named "cronstart.php" with following code
<?php
$a=system( 'crontab /fulldirectorypath/cronstat.txt',$errorvar);
echo "err=$errorvar";
echo "<BR>out=$a";
?>
Finally run "cronstart.php