Keep Form Data after php validation (POST method) - php

Question

I'm validating my form with php and displaying error messsages but after every submit , the form is refreshed and the inputs are blank again . I was wondering if there was a way i could re-insert the user past data into the fields again .
I'm using POST method .
<form id="registration-form" action="" method="post">
<input type="text" name="username" id="username" placeholder="Username">
<button type="submit" name="_submit" id="submit" class="button-submit">Submit</button>
</form>
PHP function :
if( !isset( $_POST['_submit'] ) ){
return array( $array, $arrayexport );
}
$username = $_POST['username'];
// test function for username
if ( ($valid_username ) != 0 ) {
array_push($array, "Input Required"); }
else{
header( 'Location: /thankyou.php' );
}

Answer 1

YOu can do something as
<input type="text" name="username"
id="username" placeholder="Username"
value="<?php if(isset($_POST['username'])){ echo $_POST['username'];}else{ echo '';}?>">

Answer 2

made a cleaner version .
Thanks to Abhik Chakraborty
<input type="text" name="username" id="username" placeholder="Username" value="<?php user() ?>">
External php file :
function user(){
if(isset($_POST['username'])){ echo $_POST['username'];}else{ echo '';}}