function surroundText(text1, text2, textarea)
{
   
    if (typeof(textarea.caretPos) != "undefined" && textarea.createTextRange)
    {
        var caretPos = textarea.caretPos, temp_length = caretPos.text.length;
 
        caretPos.text = caretPos.text.charAt(caretPos.text.length - 1) == ' ' ? text1 + caretPos.text + text2 + ' ' : text1 + caretPos.text + text2;
 
        if (temp_length == 0)
        {
            caretPos.moveStart("character", -text2.length);
            caretPos.moveEnd("character", -text2.length);
            caretPos.select();
        }
        else
        {
            textarea.focus(caretPos);
        }
    }
    // Mozilla text range wrap.
    else if (typeof(textarea.selectionStart) != "undefined")
    {   
        var begin = textarea.value.substr(0, textarea.selectionStart);
        var selection = textarea.value.substr(textarea.selectionStart, textarea.selectionEnd - textarea.selectionStart);
        var end = textarea.value.substr(textarea.selectionEnd);
        var newCursorPos = textarea.selectionStart;
        var scrollPos = textarea.scrollTop;
 
        textarea.value = begin + text1 + selection + text2 + end;
 
        if (textarea.setSelectionRange)
        {
            if (selection.length == 0)
                textarea.setSelectionRange(newCursorPos + text1.length, newCursorPos + text1.length);
            else
                textarea.setSelectionRange(newCursorPos, newCursorPos + text1.length + selection.length + text2.length);
            textarea.focus();
        }
        textarea.scrollTop = scrollPos;
    }
    // Just put them on the end, then.
    else
    {
        textarea.value += text1 + text2;
        textarea.focus(textarea.value.length - 1);
    }
}
 

<div class="buttons">
<input type="button" onclick="surroundText('[b]', '[/b]', document.forms.commentform.Comment934hf9v34fhjc); return false;" value="жирный">
<input type="button" onclick="surroundText('[i]', '[/i]', document.forms.commentform.Comment934hf9v34fhjc); return false;" value="курсив">
<input type="button" onclick="surroundText('[u]', '[/u]', document.forms.commentform.Comment934hf9v34fhjc); return false;" value="подчеркнутый">
<input type="button" onclick="surroundText('[html]', '[/html]', document.forms.commentform.Comment934hf9v34fhjc); return false;" value="html">
<input type="button" onclick="surroundText('[php]', '[/php]', document.forms.commentform.Comment934hf9v34fhjc); return false;" value="php">
<input type="button" onclick="surroundText('[sql]', '[/sql]', document.forms.commentform.Comment934hf9v34fhjc); return false;" value="sql">
</div>