在函数内部设置php会话-只是无法使其工作

我试图在函数中设置一个会话，但我就是不能让它工作。我已将会话开始移动到函数内部，但运气不佳

有谁能帮我解释一下为什么不设置$会话['ajax\u test\u confirm']

session_start();


function ajax_appointment_stage_one($clinic_id,$data,$tomorrow,$conn){
    $get_today_appointments = "SELECT appointment_id FROM appointments WHERE appointment_date LIKE '%$tomorrow%' AND clinic_id='$clinic_id' AND appointment_confirmed!='3' AND appointment_confirmed!='2'";
    $result = $conn->query($get_today_appointments) or die($conn->error.__LINE__);
    $num_appointments_total = $result->num_rows;
    if ($num_appointments_total==0){
    $return_value="N/A";
    } else {
    $get_today_appointments_confirmed = "SELECT appointment_confirmed FROM appointments WHERE appointment_date LIKE '%$tomorrow%' AND clinic_id='$clinic_id' AND appointment_confirmed='1'";
    $result = $conn->query($get_today_appointments_confirmed) or die($conn->error.__LINE__);
    $num_appointments_confirmed = $result->num_rows;
    $return_value=(($num_appointments_confirmed/$num_appointments_total)*100);
            $_SESSION['ajax_test_confirmed']=$return_value;
    }
    $arr=array("#".$data,$return_value);
    $return_data=json_encode($arr);
    echo $return_data;
}

$data=$_POST['div'];
$clinic_id=$_POST['clinic_id'];
$tomorrow=$_POST['tomorrow'];

include '../../includes/db_connect.php';
$data($clinic_id,$data,$tomorrow,$conn)

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当您执行此打印（$\ u会话['ajax\ u测试\ u确认]）时会发生什么？