在文件提交和手动定义的错误发生后保留输入值

//index.php

    $username=$password=$confirm_pass=$admin_type=$email="";
    $pass_exists=$con_pass_exists=$errview="";
    $errarr=array();

    ......//Verification and error push//........

    //assigning variable and error push
    if(isset($_POST["username"]) && !empty($_POST["username"])){
        $username=$_POST["username"];
    }else{
        array_push($errarr,"Username Field is Empty!!!");
    }

    if(count(errarr)>=0){
    $errview="<div class='errors'>";
      $errview.="<ul>";
      foreach ($errarr as $error){
          $errview.="<li>".$error."</li>";
      }
      $errview.="</ul>";
      $errview.="</div>";

      echo $errview;
      add_form();
    }

//html
      <form action="<?php echo htmlspecialchars($_SERVER['REQUEST_URI']) ?>"method="post" id="add_form">
          <span class="title">Username</span><input value="<?php if(isset($username))echo $username; ?>" tabindex="1" type="text" class="username" name="username">

          ......other elemenets.........
      </form>
//index.php

    $username=$password=$confirm_p