Add query string to URL if there is none already - php

Question

I have a question. My question title doesn't really describe what I need help with, but it's the same concept. This is what I want to do:
I am creating a search function (not open for other suggestions than the one I am trying to accomplish), and it's using query string. If I search for "Hello", I want my website to redirect to website.com?s=hello, but if there already is one, add it to the url with a &, like this: website.com?page=home&s=hello. I tried to use this:
if(isset($_GET)) { // Add the string with & before
else { // Add the string with a ? before
I am willing to use jQuery if I have to
More brief explanation:
I have a form, it has an input and a submit-button.
<form action="" method="post">
<li><input type="text" placeholder="Username"></li>
<li><input type="submit" value="Search"></li>
</form>
I am aware how to store everything in variables and such, but I am not sure how to add it as a query string to the url. Thank you!

Answer 1

A way to do it is to handle it in php in your form:
<?php
if(isset($_GET['page'])) {
$url = '?page=' . $_GET['page'];
}else {
$url = '';
}
?>
<form method="GET" action="<?= $url ?>" >
Search <input type="text" name="s" />
<input type="submit" value="Search" />
</form>`
Note that if you want to submit your form AND go to the page ?s=foo as a result of submission, you must have a method GET on your form and have an input with argument name="s".
EDIT: Or a bit more exhaustive, if you have several arg that could be passed in the URL it could be :
<?php
if(!empty($_GET)) {
$url = '?' . $_SERVER['QUERY_STRING'];
}else {
$url = '';
}
?>
<form method="GET" action="<?= $url ?>" >
Search <input type="text" name="s" />
<input type="submit" value="Search" />
</form>