<?php
$html = file_get_contents('https://printerservis.net/canon_V3400.php');

var_dump(mb_check_encoding($html, 'UTF-8'));

$converted = mb_convert_encoding($html, 'UTF-8', 'Windows-1254');
var_dump(mb_check_encoding($converted, 'UTF-8'));

// Let's check if there are still questions marks or if it has proper Turkish characters:
if (preg_match('/düğmesine/i', $converted)) {
    echo "SUCCESS: Found Turkish characters!\n";
} else {
    echo "FAILED to find Turkish characters in converted!\n";
    // Print first 500 chars of a text block
    if (preg_match('/<div id="wb_Text[^>]*>(.*?)<\/div>/is', $converted, $m)) {
        echo substr(strip_tags($m[1]), 0, 500) . "\n";
    }
}
