我建议的第一件事是首先使用打开：

<?php
session_start()
?>

然后插入所需的所有代码，一旦完成，您需要的是将行的值赋给变量，这样您就可以执行以下操作：

$_SESSION = $row['weight'];

因此，会话中有一个用户。如果是这样，那么您可以从 $_SESSION 变量。
要在会话中存储数据，可以执行以下操作：

<?php
session_start();
$_SESSION['user'] = $row;

现在，您的行数据在 $_SESSION 变量as user 索引。从中获取用户 $_SESSION 您可以按以下步骤操作：

<?php 
session_start();
$sessionUser = $_SESSION['user'];

在这里， session_start() 方法用于启动会话。在同一个文件中不需要两次。
然后您可以随意查看：

<?php
if($row["weight"] == $sessionUser['weight'])
  echo "matched";
else
  echo "not matched";

我给你做了一个版本，我认为这是你想要（这不是太明显）实现的。

<?php
session_start();
include "connection.php";

$id = $_GET["userID"]; // SANITISE USER INPUT!!!
        // change that to a prepared statement!! This is unsecure.
$sql = "SELECT * FROM userdetails WHERE userID = '" . $id . "'";
$result = mysqli_query($con, $sql);

if (mysqli_num_rows($result) == 1) {
    $row = mysqli_fetch_assoc($result); // no need for a while there, we expect only one row
    echo "<div>" .
        "<br>First name: " . $row["firstname"] .
        "<br>Second name: " . $row["secondname"] .
        "<br>Mobile: " . $row["mobile"] .
        "<br>Height: " . $row["height"] . "CM" .
        "<br>weight: " . $row["weight"] . "KG" .
        "<br>Image:<br> <img src=getimage.php? 
    userID=" . $row["userID"] . "width=100, height=100" .
        "<br> You are a match! click below to view events" .
        "<br><a href=Events.php?userID=" . $row["userID"] . ">View 
    Events</a>" .
        "</div>";

        // find users with the same weight;
        $findSimularity = 'weight'; // having weight as a var keeps it more modular/reuseable
        $stmt = mysqli_prepare($con, "SELECT * FROM userdetails WHERE `{$findSimularity}` = ?");

        mysqli_stmt_bind_param($stmt,"d", $row[$findSimularity]); // I assumed type double for weight here. 
                                                           //Could also be 'i' for int or 's' for a string/varchar. ?
        mysqli_execute($stmt);
        $result = mysqli_stmt_get_result($stmt);
        echo "Similar Users:<br>";
        while($similarUser = mysqli_fetch_assoc($result)) {
            echo $similarUser['firstname'] . " has the same $findSimularity of ".$row[$findSimularity]."<br>";
        }

} elseif (mysqli_num_rows($result) > 1) {
    echo "I found more than one user with given ID. That should be an error.";
} else {
    echo "0 results";
}

// best practice is to NOT have a final ?> in your files (to avoid some strange hard to find errors), so I removed it