我想在我点击提交按钮时更改定义功能的值

Question

 code.. this is a aa.php file
 <?php
 include("text.php"); 
 ?>
 <form method="post" action="text.php">
 Add product helper text: <input type="text" name="add_product"size="30" value="<?php echo $a;?>" >
<input type="submit" name="submit" value="Submit">
</form> this is a aa.php file... as i click the submit button i need to change the value in define function
<?php
define("product","function exists Return TRUE if the given function has been defined",TRUE);
$a = constant("product");
?>

this is text.php file.. when i edit the value of add_product field then the change will reflect in the define function of text.php file...like if i edit in textfield "function exists Return TRUE" then the function will become define("product","function exists Return TRUE ",TRUE);