Google Cloud Storage - Unexpected ‘Static’ (t_static) In Cacheiteminterface.PHP [PHP]

I have a bit of a strange problem. I'm trying to use Google's Storage Client to upload an image to a Google Cloud Storage bucket. But whenever I attempt to upload a picture, it gives me this error:

Parse error: syntax error, unexpected 'static' (T_STATIC) in /var/www/html/vendor/psr/cache/src/CacheItemInterface.php on line 75

Here's my code:

$privateKeyFileContent = '{--Key is here--}'

require_once 'vendor/autoload.php';
use Google\Cloud\Storage\StorageClient;

function uploadFile($bucketName, $fileContent, $cloudPath) {
    $privateKeyFileContent = $GLOBALS['privateKeyFileContent'];
    // connect to Google Cloud Storage using private key as authentication
    try {
        $storage = new StorageClient([
            'keyFile' => json_decode($privateKeyFileContent, true)
        ]);
    } catch (Exception $e) {
        // maybe invalid private key ?
        print $e;
        return false;
    }

    // set which bucket to work in
    $bucket = $storage->bucket($bucketName);

    // upload/replace file 
    $storageObject = $bucket->upload(
            $fileContent,
            ['name' => $cloudPath]
            // if $cloudPath is existed then will be overwrite without confirmation
            // NOTE: 
            // a. do not put prefix '/', '/' is a separate folder name  !!
            // b. private key MUST have 'storage.objects.delete' permission if want to replace file !
    );

    // Successful upload..?
    return $storageObject != null;
}

if(isset($_FILES['file']['tmp_name'])) {
    echo uploadFile("mybucket", file_get_contents($_FILES["file"]["tmp_name"]), "testing/" . $_FILES["file"]["name"]));
}

I'm a bit new to using the Google Cloud Storage API-- apologies if this is a silly mistake! I've been searching hours for a solution, but I haven't found any. Any help would be appreciated!