The use of brackets as to access a single character from a string is deprecated.

Use curly braces instead and you will experience the expected behavior.

i.e.
<?php
$foo    = 'bar';
$foo{2} = 't';
echo $foo;
?>

There is additional information at http://www.php.net/manual/en/language.types.string.php

Oops - that should be bogus, not closed. :)

BTW, I can't reproduce it with a recent CVS checkout:

[cynic@linux cynic]$ cat test.php 
<?
$s = "hello" ;
$s[0] = "H" ;
var_dump($s) ;
?>
[cynic@linux cynic]$ php -q test.php 
string(5) "Hello"
[cynic@linux cynic]$ php -v
4.0.7-dev

This piece of code works correct with our system as well:
<?
$s = "hello" ;
$s[0] = "H" ;
var_dump($s) ;
?>


Original sourcecode generating this problem is as follows (although i can not see a particular difference):

 function ansi2htmlsingle($t) {
 //convert special characters from one 8-byte 
 //charset another
  for ($i=0; $i<strlen($t); $i++ ) {
   $c=ord($t[$i]);
   $d=$c;
   switch ($c) {
    case 138: $d=169; break; 
    case 142: $d=174; break; 
    case 154: $d=185; break; 
    case 158: $d=190; break; 
   };
   $t[$i]=chr($d); 
  };
  return $t;  
 };

The above function is working only not for character $t[0].

This one is:

 function ansi2htmlsingle($t) {
  $r='';
  for ($i=0; $i<strlen($t); $i++ ) {
   $c=ord($t[$i]);
   $d=$c;
   switch ($c) {
    case 138: $d=169; break; 
    case 142: $d=174; break; 
    case 154: $d=185; break; 
    case 158: $d=190; break; 
   };
  $r=$r.chr($d);
  };
  return $r;  
 };

Output with second version of function:
?ITO d.o.o. Osijek

Output with first version of function:
Warning: Undefined offset: 1 in c:\inetpub\wwwroot\info.php on line 20
Warning: Undefined offset: 2 in c:\inetpub\wwwroot\info.php on line 20
etc. etc. Elements $t[1]..$t[n] are _lost_ !

changing function to start from element $t[1] (skip first character) like "for ($i=1; $i<n; $i++)" and all suddenly works fine... (except for first char not get translated)...