当我在提交表单中调用它时，我的php函数无法运行

What you need to do is commented:-

  <form method="POST" ><!-- remove  onsubmit="sida()"-->
  <textarea id="comment" placeholder="Write Your Comment Here....." name = "comment"></textarea><!-- add name attribute -->
  <br>
  <input type="text" id="username" placeholder="Your Name" name = "username"><!-- add name attribute -->
  <br>
  <input type="submit" value="Post Comment">
 </form>

  <div id="all_comments">
  <?php
    $data = array(); // define empty array
    if(isset($_POST["comment"]) && isset($_POST["username"])){ // check with posted value not button value
        $host="localhost";
        $username="root";
        $password="";
        $databasename="vinhcv_truonghoc";
        $i = 0; // DEFINE COUNTER
        $connect=mysqli_connect($host,$username,$password,$databasename); // mysql_* is deprecated so use mysqli_* or PDO
        if($connect){ // IF CONNECTION ESTABLISHED
        $comment = mysqli_real_escape_string($connect,$_POST['comment']); // Prevent from SQL Injection
        $username = mysqli_real_escape_string($connect,$_POST['username']); // Prevent from SQL Injection
            $query = mysqli_query ($connect,"INSERT INTO comments (username,comment) VALUES ('".$username."','".$comment."')"); // check and change table name as well as column name
            if($query){
                echo "Inserted Successfully";
            }else{
                echo "Problem occur in insertion because of".mysqli_error($connect);
            }
            $comm = mysqli_query($connect,"select name,comment,post_time from comments order by post_time desc");
            if($comm){ // IF QUERY EXECUTED
                while($row=mysqli_fetch_array($comm)){
                    $data[$i]["name"]       =   $row['name']; // ASSIGN VALUES TO THE ARRAY
                    $data[$i]["comment"]    =   $row['comment'];
                    $data[$i]["time"]       =   $row['post_time'];
                    $i++;
                }
            }else{
                echo "Query execution failed because of".mysqli_error($connect);
            }
        }else{
            echo'connection problem because of'.mysqli_connect_error();
        }
    }else{
        echo "All fields are need to fill properly";

    }
   ?>
    <?php foreach ($data as $dat){?> <!-- ITERATE THROUGH ARRAY -->
           <div class="comment_div"> 
              <p class="name">Posted By:<?php echo $data['name'];?></p>
              <p class="comment"><?php echo $data['comment'];?></p> 
             <p class="time"><?php echo $data['time'];?></p>
           </div>

    <?php } ?>
  </div>

Seems like you are not passing any parameters to the $_POST array. You need to pass the name attribute to your input fields. Since you are going to run a PHP server-side function you need to use the action attribute.

Like this:

 <form method="POST" action="sida()">
  <textarea name="comment" id="comment" placeholder="Write Your Comment Here....."></textarea>
  <br>
  <input name="name" type="text" id="username" placeholder="Your Name">
  <br>
  <input type="submit" value="Post Comment" name="submit">
 </form>

1.     $host="localhost";
       $username="root";
       $password="";
       $databasename="vinhcv_truonghoc";
   
   
          function connect()   {
       global $host,$username,$password,$databasename;
       $con=mysql_connect($host,$username,$password) or die ("ConnectionFailed");
       mysql_select_db($databasename,$con) or exit ("Failed to connect");
       return $con;       }    
   
   
     function iud($query) //Insert,Update,Delete   {
       $con=connect();
       $result=mysql_query($query,$con);
       $n=mysql_affected_rows($con);
       mysql_close($con);
       return $n;   }  
   
   
     function select ($query) //Select   {
       $con=connect();
       $result=mysql_query($query,$con);
       mysql_close($con);    return $result;   }   
   
   
   
   
     if(isset($_REQUEST['submit']))   {
       $name=$_REQUEST['name'];
       $comment=$_REQUEST['comment'];
       $query="INSERT INTO `comments`(`name`, `comment`, `post_time`)
       VALUES('$name','$comment',CURRENT_TIMESTAMP)";
       $n=iud($query); 
       echo $n;       }
   
     ?>
   
   
     <html>   <head></head>   <body>   <form method="GET">    <table>  
   <tr>   <td>Name</td>   <td><Input type="text" name="name" id=""
   class=""   placeholder="Name.."></td>      </tr>
          <tr>   <td>Comment</td>   <td><textarea name="comment" rows="5" cols="40" placeholder="Comment  
   
     Here.."> </textarea></td>    </tr>
          <tr>   <td></td>   <td><Input type="submit" name="submit" id="" class="" value="submit">
   
     </td>    </tr>
          </table>   </form>
          <!-- Showing Result -->     <table>   <?php   $query="Select * from comments ORDER BY coid DESC"; //Coid Is A Primary Key  
   $result=select($query);   $n=mysql_num_rows($result);   if($n>0)   {
   while($data=mysql_fetch_array($result))     extract($data);   }    ?>  
   <tr>       <td><?php echo @$name; ?></td>      </tr>   <tr>   <td><?php
   echo @$comment; ?></td>    </tr>   </table>
              </body>     </html>
   

   I hope It will Work For you :)