PHP form validation problem?

I'm making one form using php, so validation is using PHP too,
I m posting the value in the same form and then trying to insert into mysql..look below.. I encountered the problem
--------------------------------------…

<form method="post" enctype="multipart/form-data" name="form1" id="form1">

Name : <input type="text" name="t1" id="t1" />
Contact : <input type="text" name="t2" id="t2" />

<span class="err" ></span>

<input type="submit" name="s1" id="s1" />  
</form>

<?php
if(isset($_POST["s1"]))
{
if($_POST["t1"] == "")
{
$err = "Enter Your name";

echo "<script language=javascript>" . "alert('Enter your name')" . "</script>";
echo "<script language=javascript>" . "document.getElementById('t1').focus();" . "</script>";
}
else if($_POST["t2"] == "")
{
$err = "Enter Your Contact Number";
echo "<script language=javascript>" . "alert('Enter your contact number')" . "</script>";
}

echo "<span style=color:red> $err</span>";
}

?>

--------------------------------------…

now what problem m facing here when i click on the "Submit" button i m getting the Error messages but the page is redirecting the i have to write all values all over again ..

suppose i have already wrriten a "NAME(t1)" but didnt write a CONTACT NUMBER(t2) <a href="https://www.phpkode.com/scripts/category/php-validation/" target="_blank" rel="nofollow noopener noreferrer">Новое порно: Двойное проникновение. Cмотреть онлайн, бесплатно скачать</a> then after m clicking on the SUBMIT button when i do this my page redirecting, m getting the second error "write your contact number" but i have to write all the values like NAME etc all over again ..

so my page is returning to true .. not false .

do you know how to set "return false" through php just like we do using javascript .. i cant use javascript for this .. have to use php in this .. plz help me ..

thank you ..