Learning MySQL

Exercises

1. In a PHP script, how can we access data entered by a user in an HTML form?

2. Why is it important to untaint information that arrives from the client?

3. What is the difference between the PHP library functions mysqli_num_rows() and mysqli_affected_rows()?

4. When would you pass a variable to a function by reference rather than by value?

5. What does this program do?

   #!/usr/bin/php
   <?php
     $Time="2006-06-20 19:00:00";
     TimeDifference($Time, $seconds, $hours, $minutes, $days);
     echo "\n".sprintf("%3d days, %2d hours, %2d minutes, and %2d seconds",
         $days, $hours, $minutes, $seconds);
     echo " since you left ... :(";
   
     $Time="2006-11-15 20:00:00";
     TimeDifference($Time, $seconds, $hours, $minutes, $days);
     echo "\n".sprintf("%3d days, %2d hours, %2d minutes, and %2d seconds",
         $days, $hours, $minutes, $seconds);
     echo " till I see you again... :)";
   
     echo "\n";
   
     function TimeDifference($ReferenceTime, &$seconds, &$hours, &$minutes, &$days)
     {
       $seconds=abs(strtotime($ReferenceTime) - mktime());
       $days =intval(($seconds)                                   /( 24*60*60));
       $hours =intval(($seconds-($days*24*60*60))                 /(    60*60));
       $minutes=intval(($seconds-($days*24*60*60)-($hours*60*60)) /(       60));
       $seconds=intval(($seconds-($days*24*60*60)-($hours*60*60)-($minutes*60)));
     }
   ?>

   You should use the PHP manual at http://www.php.net/manual/ to look up information on new functions. To get you started, here are some short explanations:

   abs()

   Returns the absolute value of a number passed to it (it removes the minus sign for negative numbers).

   intval()

   Converts ...