Hi I have this PHP Code -

<?php
require_once ("../include/initialize.php");
     if (!isset($_SESSION['ACCOUNT_ID'])){
      redirect(web_root."index.php");
     }

$action = (isset($_GET['action']) && $_GET['action'] != '') ? $_GET['action'] : '';

switch ($action) {
    case 'add' :
    doInsert();
    break;

    case 'edit' :
    doEdit();
    break; 

    case 'delete' :
    doDelete();
    break;

    case 'photos' :
    doupdateimage();
    break;

    case 'checkid' :
    Check_StudentID();
    break;

    }

    function doInsert(){
        if(isset($_POST['save'])){


        if ($_POST['StudentID'] == "" OR $_POST['Firstname'] == "" OR $_POST['Lastname'] == ""
            OR $_POST['Middlename'] == "" OR $_POST['CourseID'] == "none"  OR $_POST['Address'] == "" 
            OR $_POST['ContactNo'] == "") {
            $messageStats = false;
            message("All fields are required!","error");
            redirect('index.php?view=add');
        }else{  

            $birthdate =  $_POST['year'].'-'.$_POST['month'].'-'.$_POST['day'];

            $age = date_diff(date_create($birthdate),date_create('today'))->y;

            if ($age < 15){
            message("Invalid age. 15 years old and above is allowed.", "error");
            redirect("index.php?view=add");

            }else{
                // error message 
                // duplicate student id
                 // date_format(date_create($_POST['BirthDate']),'Y-m-d'); 

                $sql = "SELECT * FROM tblstudent WHERE StudentID='" .$_POST['StudentID']. "'";
                $res = mysql_query($sql) or die(mysql_error());
                $maxrow = mysql_num_rows($res);
                if ($maxrow > 0) { 
                    # code... 
                    message("Student ID already in use!", "error");
                    redirect("index.php?view=add");
                }else{
                    $stud = New Student(); 
                    $stud->StudentID        = $_POST['StudentID'];
                    $stud->Firstname        = $_POST['Firstname']; 
                    $stud->Lastname         = $_POST['Lastname'];
                    $stud->Middlename       = $_POST['Middlename'];
                    $stud->CourseID         = $_POST['CourseID']; 
                    $stud->Address          = $_POST['Address']; 
                    $stud->BirthDate        = $birthdate;
                    $stud->Age              = $age;
                    $stud->Gender           = $_POST['optionsRadios']; 
                    $stud->ContactNo        = $_POST['ContactNo'];
                    $stud->YearLevel        = $_POST['YearLevel'];
                    $stud->create();

                                // $autonum = New Autonumber();  `SUBJ_ID`, `SUBJ_CODE`, `SUBJ_DESCRIPTION`, `UNIT`, `PRE_REQUISITE`, `COURSE_ID`, `AY`, `SEMESTER`
                                // $autonum->auto_update(2);

                    message("New student created successfully!", "success");
                    redirect("index.php");

                }

            }
         }
        }

    }

    function doEdit(){
    if(isset($_POST['save'])){

        if ($_POST['StudentID'] == "" OR $_POST['Firstname'] == "" OR $_POST['Lastname'] == ""
        OR $_POST['Middlename'] == "" OR $_POST['CourseID'] == "none"  OR $_POST['Address'] == "" 
        OR $_POST['ContactNo'] == "") {
            $messageStats = false;
            message("All fields are required!","error");
            redirect('index.php?view=add');
        }else{  

            $birthdate =  $_POST['year'].'-'.$_POST['month'].'-'.$_POST['day'];

            $age = date_diff(date_create($birthdate),date_create('today'))->y;
            if ($age < 15){
               message("Invalid age. 15 years old and above is allowed.", "error");
               redirect("index.php?view=view&id=".$_POST['StudentID']);

            }else{

          // echo  $_POST['optionsRadios']; 
                $stud = New Student(); 
                $stud->StudentID        = $_POST['IDNO'];
                $stud->Firstname        = $_POST['Firstname']; 
                $stud->Lastname         = $_POST['Lastname'];
                $stud->Middlename       = $_POST['Middlename'];
                $stud->CourseID         = $_POST['CourseID']; 
                $stud->Address          = $_POST['Address']; 
                // $stud->BirthDate     = date_format(date_create($_POST['BirthDate']),'Y-m-d');  
                $stud->BirthDate        = $birthdate;
                $stud->Age              = $age;
                $stud->Gender           = $_POST['optionsRadios']; 
                $stud->ContactNo        = $_POST['ContactNo'];
                $stud->YearLevel        = $_POST['YearLevel'];

                $stud->studupdate($_POST['StudentID']);



                message("Student has been updated!", "success");
                redirect("index.php?view=view&id=".$_POST['StudentID']);
            }


        }


    }

} 

    function doDelete(){

        if (isset($_POST['selector'])==''){
        message("Select the records first before you delete!","error");
        redirect('index.php');
        }else{

        $id = $_POST['selector'];
        $key = count($id);

        for($i=0;$i<$key;$i++){

            $subj = New Student();
            $subj->delete($id[$i]);


                // $id =    $_GET['id'];

                // $subj = New Student();
            //      $subj->delete($id);


        }
            message("Student(s) already Deleted!","success");
            redirect('index.php');
        }


    }
    function doupdateimage(){

            $errofile = $_FILES['photo']['error'];
            $type = $_FILES['photo']['type'];
            $temp = $_FILES['photo']['tmp_name'];
            $myfile =$_FILES['photo']['name'];
            $location="photo/".$myfile;


        if ( $errofile > 0) {
                message("No Image Selected!", "error");
                redirect("index.php?view=view&id=". $_GET['id']);
        }else{

                @$file=$_FILES['photo']['tmp_name'];
                @$image= addslashes(file_get_contents($_FILES['photo']['tmp_name']));
                @$image_name= addslashes($_FILES['photo']['name']); 
                @$image_size= getimagesize($_FILES['photo']['tmp_name']);

            if ($image_size==FALSE ) {
                message("Uploaded file is not an image!", "error");
                redirect("index.php?view=view&id=". $_GET['id']);
            }else{
                    //uploading the file
                    move_uploaded_file($temp,"photo/" . $myfile);



                        $stud = New Student();
                        $stud->StudPhoto    = $location;
                        $stud->studupdate($_POST['StudentID']);
                        redirect("index.php?view=view&id=". $_POST['StudentID']);


                    }
            }

        }

    function Check_StudentID(){


        // $stud = New Student();  
        // $res = $stud->single_student($_POST['IDNO']);

        $sql = "SELECT * FROM tblstudent WHERE StudentID='" .$_POST['IDNO']. "'";
        $res = mysql_query($sql) or die(mysql_error());
        $maxrow = mysql_num_rows($res);
        if ($maxrow > 0) { 
            # code...
            echo "Student ID already in use!"; 
        }


    }

?>

And I am getting this error - Fatal error: Uncaught Error: Call to undefined function mysql_query() in C:\xampp\htdocs\attendancemonitoring\student\controller.php:58 Stack trace: #0 C:\xampp\htdocs\attendancemonitoring\student\controller.php(11): doInsert() #1 {main} thrown in C:\xampp\htdocs\attendancemonitoring\student\controller.php on line 58

I needed help im new .

Sorry but I see the comment in an alert but not here. Sometimes folk try to copy code from the web without adjusting to their system.

You asked "do i need to add the code from the link u send to my code ?"

The answer is that you need to add code to connect and select the database you intend to use. You can't query until that step is done. You can't just copy and paste code from the web.

For first check your MySQL version

select version() from dual;

Because all "mysql_..." in current (MySQL 8.0.) is deprecated - probably you should be use "mysqli_..." instead

I'm sorry - check your PHP version. Extension "mysql_..." depraceted in PHP 5.5.0 and removed in PHP 7 mysql_query

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