'static variables in php not holding value

I have written a demo program to describe a problem I'm having with static variables in php. I have a singleton class, History.php, that contains a static $idx variable intially set to -1. An incIdx() function in the History class increments $idx. The demo starts by calling index.php which gets an instance of History, repeatedly calls incIdx() and displays the $idx final value. Then, on clicking a button, test.php is called which gets the History instance, repeatedly calls incIdx(), and displays the final value. What I don't understand is, the value created by index.php is not the starting value for the test.php. That is, index.php and test.php both start with the initial value = -1. The code follows:

<?php
class History {
    private static int $idx;
    private static $instance = null;
    
    private function __construct() {
        $this::$idx = -1;
    }
    
    public static function getHistory() {
        if (self::$instance === null) {
            self::$instance = new History();
        } 
        return self::$instance;
    }
    
    public function incIdx() {
        $this::$idx++;
    }
    
    public function getIdx(): int {
        return $this::$idx;
    }
}

<?php
require_once "History.php";

$instance = History::getHistory();
for ($i = 0; $i < 5; $i++) {
    $instance->incIdx();
}
$value = $instance->getIdx();
?>
<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>Test Static</title>
    </head>
    <body>
        <h1>In index.php</h1>
        <h2>Current idx = "<?php echo $value; ?>"</h2>
        <button type="button"><a href="test.php">Click to continue</a></button>
    </body>
</html>

<?php
require_once "History.php";

$instance = History::getHistory();
for ($i = 0; $i < 5; $i++) {
    $instance->incIdx();
}
$value = $instance->getIdx();
?>
<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>Test Static</title>
    </head>
    <body>
        <h1>In test.php</h1>
        <h2>Current idx = "<?php echo $value; ?>"</h2>
    </body>
</html>```

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'static variables in php not holding value

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