Calculate total price of two books using PHP and HTML -

i want know can way using php , html such provided dropdown of books, need select 2 choices out of drop down , calculate sum of price of both books.

assuming had hardcoded books say:

book 1 - $5 book 2 - $15 book 3 - $50 

i know if select 1 book. no idea one. please help

code :

<?php if(isset($_post['formsubmit']))  {     $varcurrentbook = $_post['formbook'];     $errormessage = "";      if(empty($varcurrentbook))      {         $errormessage = "<li>you forgot select book!</li>";     }      if($errormessage != "")      {         echo("<p>there error form:</p>\n");         echo("<ul>" . $errormessage . "</ul>\n");     }      else      {         switch($varcurrentbook)         {             //can use here find option clicked         }         exit();     } } ?> <form action="<?php echo htmlentities($_server['php_self']); ?>" method="post">     <label for='formbook'>select book</label><br>     <select name="formbook">         <option value="0">select book...</option>         <option value="15">the secret</option>         <option value="10">the fairy tales</option>         <option value="5">all words</option>         <option value="100">pinaacle studio</option>         <option value="120">harry potter</option>         <option value="200">thinking in java</option>    </select>     <input type="submit" name="formsubmit" value="submit" /> </form> 

you need use multiple select as:

<form action="<?php echo htmlentities($_server['php_self']); ?>" method="post"> <label for='formbook'>select book</label><br> <select name="formbook[]" multiple><!--the multiple attribute-->     <option value="0">select book...</option>     <option value="15">the secret</option>     <option value="10">the fairy tales</option>     <option value="5">all words</option>     <option value="100">pinaacle studio</option>     <option value="120">harry potter</option>     <option value="200">thinking in java</option> </select>  <input type="submit" name="formsubmit" value="submit" /> </form> 

also note how changed name formbook formbook[]. changes $_post['formbook'] array of options.

then can access as:

<?php   foreach ($_post['formbook'] $names)   {     print "you have selected $names<br/>";     /*your need change code here accordingly.*/   }  ?>