if(isset($_POST['corse_nm'])){
    // $sql_sel=mysqli_query($conn,"SELECT * FROM regis WHERE course ='$key' ");    
    
    if($_POST['corse_nm'] == 'all'){
        $sql_sel=mysqli_query($conn,"SELECT * FROM regis");
    }
    
    elseif($_POST['corse_nm'] != ''){ 

        // if($_POST['corse_nm'] != ''){
        //     echo "1";
            $sql_sel=mysqli_query($conn,"SELECT * FROM regis WHERE course ='".$_POST['corse_nm']."'");
        // }
    }
        elseif(!empty($_POST['corse_nm']) and $_POST['mrks_nm'] > 50 ){
            echo "2";
            $sql_sel=mysqli_query($conn,"SELECT * FROM regis WHERE course ='".$_POST['corse_nm']."' and marks > 50 ");
        }
        elseif($_POST['corse_nm'] = 'isselected' || $_POST['mrks_nm'] < 50 ){
            echo "3";
            $sql_sel=mysqli_query($conn,"SELECT * FROM regis WHERE course ='".$_POST['corse_nm']."' and marks < 50 ");
        }
        else{
            $sql_sel=mysqli_query($conn,"SELECT * FROM regis WHERE course ='".$_POST['corse_nm']."'");
        }

以上代码用于课程下拉和标记下拉，如何签入条件？

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