PHP: still work even if not isset

HTML new_password:

   <input name="new_password" type="password" maxlength="25" id="new_password" class="ModuleTextbox" onkeyup="var regex = new RegExp(/^.*(?=.{6,})(?=.*\d)(?=.*[aA-zZ@!¤&amp;/()_?,.-]).*$/m); if(regex.test(this.value)) { pwok.style.visibility = 'visible'; } else { pwok.style.visibility = 'hidden'; }" style="width:200px;" />&nbsp;<img id="pwok" src="ok.png"
                            alt="R" style="visibility: hidden;" />

php after submit form:

foreach开发者_Go百科($_POST as $key => $value) {
    $data[$key] = filter($value);
}
$pw_new = $data["new_password"];


   if(isset($pw_new)) {
echo "LOL";
}

even if i didnt write anything in the field, it echo's LOL, why's that?

Because the input is there, the form will still send it even if it's empty. So even if it's empty, it's still set, as in it still exists in $_POST, and so it'll still carry over to $data.

You should use !empty() (that means 'not empty') instead:

if(!empty($pw_new)) {
}

Even when you write nothing, you foreach will always set $data["new_password"], then it set $pw_new, you might want so check for values instead of isset

if ($pw_new != '') {
echo 'LOL';
}