Uninitialized string offset php - php

Question

I am writting my own calculator on PHP.
I have a problem with my code because i don't know where i am trying to read too far in the string. So if anyone can enlighten me ..
The exact error i get is :
PHP Notice: Uninitialized string offset: 4 in /home/salim/Bureau/web/piscine_php/d01/ex11/do_op_2.php on line 76
Here is the code below :
function decoupe ($argv)
{
global $nbr1;
global $nbr2;
global $sign;
$string = NULL;
$string = trim($argv[1], " \t");
echo $string;
echo "\n";
$x = 0;
while($string[$x])
{
if (is_numeric($string[0]) == false)
error_msg();
if (is_numeric($string[$x]) && $string[$x + 1])
{
while (is_numeric($string[$x]))
{
$nbr1 .= $string[$x];
$x++;
}
}
if (is_thisoperator(substr($string, $x)))
{
$sign .= $string[$x];
$x++;
}
else
{
error_msg();
}
if ($string[$x + 1] && is_numeric($string[$x]))
{
while (is_numeric($string[$x]))
{
$nbr2 .= $string[$x];
$x++;
}
}
else
{
error_msg();
}
}

Answer 1

Don't use $string[$x] as a way to test whether $x is a valid index in the string. It prints a warning when $x is outside the string. Use $x < strlen($string) instead. So change:
while ($string[$x])
to
while ($x < strlen($string))
and change
if ($string[$x + 1] && is_numeric($string[$x]))
to
if ($x + 1 < strlen($string) && is_numeric($string[$x]))