php : Notice: Use of undefined constant

I receive this error on my page..

Notice: Use of undefined constant one - assumed 'one' in

How can to fix this??

Answer:
Let's see your code

error_reporting(2047); 
$a=array(1=>'ONE','one'=>1);
$s.=$a[one]."<br>";

die($s);

As we see. One should be type $a['one'] or this will happen: FYI the right answer is 1 not ONE

the code are like this

<?php
error_reporting(2047);
define('one',1);
$a=array(1=>'ONE','one'=>1);
$s.=$a[one]."<br>";
$s.=$a['one']."<br>";

die($s);

If you define one (or perhaps another script).. The result is not 1 but ONE. For the next time.. LET's More carefully..
Another way if you can't edit all and don't have time.. edit

error_reporting(2047);

into (if there is no reporting.. add it)

error_reporting(0);