php - 当尝试在页面的两个部分中使用 PHP 时，为什么会出现 undefined variable 错误？

Question

我尝试在页面的两个不同部分使用 PHP。但是当我尝试加载页面时，我收到错误: undefined variable $conn。为什么我会收到此错误？

<tbody>
<?php
     $sql = "SELECT id, name, price FROM periperichicken";
     $result = mysqli_query($conn, $sql);
     if (mysqli_num_rows($result) > 0) {
         // output data of each row
         while($row = mysqli_fetch_assoc($result)) {
             echo "<tr>";
             echo "<td>" . $row["name"] . "</td>";
             echo "<td>£" . $row["price"] . "</td>";
             echo "<td><input type=\"text\" name=\"amount\" 
                 class=\"amount-type\"
                 placeholder=\"Amount\"/></td>";
             echo "<td><a href=\"\" class=\"add-cart\">Add to cart</a></td>";
             echo "</tr>";
         }
     }
     mysqli_close($conn);
?> 
</tbody>

<?php
    $servername = "localhost";
    $username = "root";
    $password = "";
    $dbname = "pizza";

    // Create connection
    $conn = mysqli_connect($servername, $username, $password, $dbname);
    // Check connection
    if (!$conn) {
        die("Connection failed: " . mysqli_connect_error());
    }
?>

Answer 1

您在脚本顶部使用 $conn

$result = mysqli_query($conn, $sql);

但是你进一步定义它

$conn = mysqli_connect($servername, $username, $password, $dbname);

您需要将该部分移至第一个部分，然后才能运行任何查询。

<?PHP
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "pizza";

// Create connection
$conn = mysqli_connect($servername, $username, $password, $dbname);
// Check connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}
?>

<tbody>
<?php
  $sql = "SELECT id, name, price FROM periperichicken";
  $result = mysqli_query($conn, $sql);

  if (mysqli_num_rows($result) > 0) {
    // output data of each row
    while($row = mysqli_fetch_assoc($result)) {
      echo "<tr>";
      echo "<td>" . $row["name"] . "</td>";
      echo "<td>£" . $row["price"] . "</td>";
      echo "<td><input type=\"text\" name=\"amount\" class=\"amount-type\" placeholder=\"Amount\"/></td>";
      echo "<td><a href=\"\" class=\"add-cart\">Add to cart</a></td>";
      echo "</tr>";
    }
  }

  mysqli_close($conn);
?> 
</tbody>