语法错误意外'？' 在当地工作[关闭]

i have this code in the top of my index.php. And it works perfect when i run it locally with mamp. But when i go live with the page and upload it to the hosting service and launch the website i get this error in the log. And the site does not load.

PHP Parse error: syntax error, unexpected '?' in /home/*****/*****.*****.**/index.php on line 3

<?php
session_start();
$AccountsucessRegister = $_SESSION['Accountsucess'] ?? '';
$AccountfailRegister  = $_SESSION['Accountfail'] ?? '';
unset($_SESSION['Accountsucess']);
unset($_SESSION['Accountfail']);
?>
 <?php if ($AccountsucessRegister !== ''): ?>
 <?php echo "<script>alert('$AccountsucessRegister');</script>" ?>
 <?php endif; ?>
 <?php if ($AccountfailRegister !== ''): ?>
 <?php echo "<script>alert('$AccountfailRegister');</script>" ?>
 <?php endif; ?>

Maybe you have on local php7+ version and at your provider not.

Among provider have .ini file option to change the php version on shared hosting but if not the error comes from

$AccountsucessRegister = $_SESSION['Accountsucess'] ?? '';

must be

$AccountsucessRegister = isset($_SESSION['Accountsucess']) ? $_SESSION['Accountsucess'] : '';