PHP doesn't preform calucation -

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i've php form user enters height , weight , once click submit info resubmitted form , calucation bmi should excute , display result. i'm little unsure how code , i've checked errors phpcodechecker.com found none, don't result. i'm unsure if calculation happening @ all. on how calculate bmi off info user enters appreciated.

the forumal need use

bmi = (weight in pounds * 703) / (height in inches) squared note: there 12 inches foot so, if person weighs 150 pounds , 6 feet tall, calculation bmi = (150 * 703) / 72 x 72 (6 feet = 72 inches, squared)

code below: 

  <form method="post" action="<?php echo $_server['php_self']; ?>">     <label for="weight">enter weight in pounds:</label><br>     <input id="weight" name="weight" type="text" value="<?php echo $weight; ?>" size="30"><br>     <label for="height">enter height in inches:</label><br>     <input id="height" name="height" type="text" value="<?php echo $height; ?>" size="30"><br>     <input type="submit" name="submit" value="calculate">   </form>  <?php   } ?> <?php $weight = ''; $height = '';    if (isset($_post['submit'])) {     $weight = $_post['weight'];     $height = $_post['height'];     $output_form = false;      if (empty($weight) && empty($height)) {       echo 'you forgot enter weight.<br />';       $output_form = true;     }      if (empty($weight) && (!empty($height))) {       echo 'you forgot enter height.<br />';       $output_form = true;     }   }   else {     $output_form = true;   }    if ((!empty($weight)) && (!empty($height))) {       $string = "(weight * 703)/height * height";       eval($string);       echo($res); ?>   <form method="post" action="<?php echo $_server['php_self']; ?>">     <label for="weight">enter weight in pounds:</label><br>     <input id="weight" name="weight" type="text" value="<?php echo $weight; ?>" size="30"><br>     <label for="height">enter height in inches:</label><br>     <input id="height" name="height" type="text" value="<?php echo $height; ?>" size="30"><br>     <input type="submit" name="submit" value="calculate">   </form>  <?php   } ?>

some tips:

  • avoid using eval in code. slow , insecure.
  • it seems have same form 2 times (repeated code). also, try avoid this. changed code don't have repeat it

a better working code may this:

<?php $weight = ''; $height = ''; $output_form = true;  if (isset($_post['submit'])) {      $weight = $_post['weight'];     $height = $_post['height'];      if (empty($weight) && empty($height)) { //test both         echo 'you forgot enter weight , height.<br>';     }else if (empty($weight)) {         echo 'you forgot enter weight.<br />';     }else if (empty($height)) {         echo 'you forgot enter height.<br />';     }else{ //if none empty, can proceed calc         $output_form = false; //everything ok, hide form         $res = ($weight * 703)/($height * $height);         echo $res;     } }  if($output_form) { ?>      <form method="post" action="<?php echo $_server['php_self']; ?>">         <label for="weight">enter weight in pounds:</label><br>         <input id="weight" name="weight" type="text" value="<?php echo $weight; ?>" size="30"><br>         <label for="height">enter height in inches:</label><br>         <input id="height" name="height" type="text" value="<?php echo $height; ?>" size="30"><br>         <input type="submit" name="submit" value="calculate">     </form>  <?php }

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