How to insert $_POST in HTML form input?

Question

How to insert a $_POST in a input of a form HTML? In a simple login form, when leaving a blank field, the script rescues the filled field, keeps it filled, and informs the empty field error, so that the user does not have to type again.

There’s a way to do this with PHP, without Javascript? Basically the script would receive the $_POST sent and filled in the respective field.

Only the insertion script is missing anyway.

<?php
function checkUsername($username)
{
	if ($username == "")
	{
		echo "<span style='color:red'>Preencha o campo Username!</span><br>";
	}
}

function checkPassword($password)
{
	if ($password == "")
	{
		echo "<span style='color:red'>Preencha o campo Password!</span><br>";
	}
}

<?php
include 'includes/functions.php';

$check = false;

if ($_SERVER['REQUEST_METHOD'] == 'POST')
{
	checkUsername($_POST['username']);
	checkPassword($_POST['password']);
	$check = true;
}
?>

<!-- FORMULÁRIO DE LOGIN -->
<div id="form">
	<form action="" method="post">
		<div class="_100">
			<label for="username">Username: </label><br>
			<input type="text" name="username" placeholder="Username"><br>
		</div>
		<div class="">
			<label for="password">Password: </label><br>
			<input type="password" name="password" placeholder="*******"><br>
		</div>
		<br>
		<div class="">	
			<input type="submit" value="Entrar no Sistema">
		</div>
	</form>
</div>
<!-- /FORMULÁRIO DE LOGIN -->

Answer 1

use the function filter_input() within the attribute value:

<input type="text" name="username" placeholder="Username" value="<?php
  echo filter_input('username', INPUT_POST);
?>">
<input type="password" name="password" placeholder="*******"/>

The code is equivalent to echo $_POST['username'], except by checking to avoid errors of the function itself.

Important: do not put the entered password in the form because it can expose the user data.

Answer 2

Hello no need to even write php for this, just put the fields require="" that it is an attribute of HTML 5 so if there is no data inserted or will pass clear depending on the browser version:

<div id="form">
<form action="" method="post">
    <div class="_100">
        <label for="username">Username: </label><br>
        <input type="text" name="username" placeholder="Username" require=""><br>
    </div>
    <div class="">
        <label for="password" require="">Password: </label><br>
        <input type="password" name="password" placeholder="*******" require=""><br>
    </div>
    <br>
    <div class="">  
        <input type="submit" value="Entrar no Sistema">
    </div>
</form>

I believe that it works and will keep the code much more succinct, now if you do not want to do so you can create session variables or put in GET or POST variables directing to the same page, an example of GET variable would be:

<?php
if(isset($_GET["nome"])) {
    $nome   = $_GET["nome"];
} else {
   nome="";
}
  //e em seu input

?>

<input value="<?=$nome;?>" type="text" name="nome">