How do I get a fully decoded string - php

Question

I am using $_SERVER['QUERY_STRING'] instead of $_GET to get all my parameters. However, $_SERVER['QUERY_STRING'] encodes the string by itself. Thus, if a user sends an encoded string, it gets encoded again with $_SERVER['QUERY_STRING']. Meaning I have to run urldecode twice.
Encoded string: https://www.example.com/test.php?info1234=3177%3B315961%3B317451%3B315511&info3598=121618%3B136803%3B13830%3B20532
If you use the link above to submit via the URL itself. Meaning you put the parameter in yourself, you get the string (which is what I want):
https://www.example.com/test.php?info1234=3177;315961;317451;315511&info3598=121618;136803;13830;20532
However, if you submit via form, you get a doubly encoded string. So when you decode, it only decodes once, and you have to run urldecode on it again to fully decode it:
https://www.example.com/test.php?info1234=3177%3B315961%3B317451%3B315511&info3598=121618%3B136803%3B13830%3B20532
How can I make sure I get the decoded string (I don't want to run urldecode twice on the string, as it makes my time complexity O(2n). I want to try keep it at O(1) or at the very least O(n). I know using urldecode means n every time.
<?php
$a = urldecode($_SERVER['QUERY_STRING']);
$b = urldecode($a);
if(strpos($a, "%") != false) {
echo "We had to decode twice<br>";
echo $a . "<br><br>Then to:<br>" . $b;
} else {
echo "Only decoded once!!<br>";
echo $a;
}
?>
<form action="test.php" method="get">
myurl: <input type="text" name="myurl"><br>
<input type="submit" value="Submit">
</form>
Method 1:
localhost/test.php?https://www.example.com/test.php?info1234=3177%3B315961%3B317451%3B315511&info3598=121618%3B136803%3B13830%3B20532
This decodes once
Method 2:
Go to localhost/test.php
Submit this link via the form
https://www.example.com/test.php?info1234=3177%3B315961%3B317451%3B315511&info3598=121618%3B136803%3B13830%3B20532
This decodes twice
You will see you get 2 different outputs