preg_match_all('/<h[1-6]>(.*?)<\/h[1-6]>/', $content, $cikti);
foreach($cikti[1] as $title) {
    echo '<a href="#' . seo($title) . '" onclick="goAnchor(' . seo($title) . ')">' . $title . '</a>' . "<br>";
}
// muhtemel hata olan yer
foreach ($cikti[0] as $key => $element) {
    $title = $cikti[1][$key];
 
    $newElement = '<h1 id="' . seo($title) . '">' . $title . '</h1>';
    $content = str_replace($element, $newElement, $content);
 
    $newElement = '<h2 id="' . seo($title) . '">' . $title . '</h2>';
    $content = str_replace($element, $newElement, $content);
 
    $newElement = '<h3 id="' . seo($title) . '">' . $title . '</h3>';
    $content = str_replace($element, $newElement, $content);
 
    $newElement = '<h4 id="' . seo($title) . '">' . $title . '</h4>';
    $content = str_replace($element, $newElement, $content);
 
    $newElement = '<h5 id="' . seo($title) . '">' . $title . '</h5>';
    $content = str_replace($element, $newElement, $content);
 
    $newElement = '<h6 id="' . seo($title) . '">' . $title . '</h6>';
    $content = str_replace($element, $newElement, $content);
}