php - Almost the same code, but different output, why?

Solution:

You are reading the file byte at a time.

For example the characterб encodes as bytes0xD0 0xB1 in UTF-8. The tab character is0x09.

So without the tab character, you first write0xD0, then0xB1, resulting in0xD0 0xB1 which is valid UTF-8.

With the tab character, you write0x09 between every byte - making it:0xD0 0x09 0xB1.0xD0 followed by0x09 is not valid UTF-8, so the browser renders the replacement character to deal with it.

You need to be more sophisticated about it; this should work:

$file = fopen("t1.txt","r+");
while (! feof ($file))
{
  $c = fgetc($file);
  $val = ord($c);

  //UTF-8 Lead Byte
  if( $val & 0x80 ) {
    $continuationByteCount = 0;
    if( ($val & 0xF8) == 0xF0) $continuationByteCount = 3;
    else if( ($val & 0xF0) == 0xE0) $continuationByteCount = 2;
    else if( ($val & 0xE0) == 0xC0) $continuationByteCount = 1;

    echo $c;

    while( $continuationByteCount-- ) {
        echo fgetc($file);
    }

  }
  else { //Single-byte UTF-8 unit... I.E. ASCII
      echo $c;
  }
  echo "\t";
}

fclose($file);
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Read it all at once and split to array where each item is 1 character (1-4 bytes):

$chars = preg_split( '//u', file_get_contents("t1.txt"), -1, PREG_SPLIT_NO_EMPTY );

foreach( $chars as $char ) {
    echo $char;
    echo "\t";
}
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