Default value to insert into MySQL

[Jaguar]

What should I insert as the default value if a form field is blank? My fields don't currently accept null.

 

if($_POST['height']) {
  // error checking
  $height = $_POST['height'];
}
else {
  $height = '';
}

 

Should $height be equal to '', NULL, or maybe 'NULL'? I'm thinking 'NULL' incase I or someone else decides to change the field to accept null?

[Caesar]

<?php

  function check_value($myvalue) {

    if((!isset($_POST['.$myvalue.'])) | ($myvalue == '')) {
    
      $myvalue = '';
    }

      else {
      $myvalue = $clean->clean_input($_POST['.$myvalue.']);
      }

  return $myvalue;
  }

?>