Something wrong with array - php

Question

I'm using this code to get first number and replace it with 0-9, but I always get 0-0-9 result. Then I deleted in array 9 it started to work correct. why it works that way ?
$direction = strtoupper(substr($query_row["Band"],0,1));
$replace = [
'0' => '0-9','1' => '0-9','2' => '0-9','3' => '0-9','4' => '0-9',
'5' => '0-9','6' => '0-9','7' => '0-9','8' => '0-9', '9' => '0-9' ];
$dir= str_replace(array_keys($replace), $replace, $direction);

Answer 1

try this one
$search = array('0','1','2','3','4','5','6','7','8');
$replace = array('0-9','0-9','0-9','0-9','0-9','0-9','0-9','0-9','0-9');
$dir = str_replace($search, $replace, $direction);
and then work around for 9 which depends on your string
mine was 0123456789 so I tried
$dir = str_replace('99', '9,0-9', $dir);
its working on mine

Answer 2

It's explained in the documentation of str_replace():
Caution
Replacement order gotcha
Because str_replace() replaces left to right, it might replace a previously inserted value when doing multiple replacements.
You pass arrays as first two arguments to str_replace(). This is the same as you would call str_replace() repeatedly for each element in the array.
If $direction is not '9' then it replaces it with '0-9'. Then, on the last cycle it replaces '9' from '0-9' with '0-9' (according to the values you passed it).
I would drop the code after the first line and read the first sentence again: "get first number and replace it with 0-9".
If your goal is to get the first character of $query_row["Band"] and replace it with 0-9 if it is a digit (and make it uppercase otherwise) I would write something more simple:
$direction = substr($query_row["Band"], 0, 1);
if (is_numeric($direction)) {
$dir = '0-9';
} else {
$dir = strtoupper($direction);
}