程序文件中的任何代码都将在执行时运行，这对所有语言都适用，除非它 Package 在 if 或任何其他条件陈述；在这种情况下，条件语句中包含的代码只有在条件为 true .
因此，为了解决您的问题，您需要一个条件语句来检查表单是否已提交，然后执行下面的代码-

if(isset($_POST['submit'])){
    $host = "localhost";
    $db = "cis475";
    $user = "root";
    $pw = "";

    $conn = new mysqli ($host, $user, $pw, $db);
    if($conn->connect_error) die($conn->connect_error);

    $readAllQuery = "SELECT * FROM enrollment JOIN course ON enrollment.CourseID = course.CourseID JOIN student ON enrollment.StudentID = student.StudentID";

    $result = $conn->query($readAllQuery);
    if (!$result) die($conn->error);

    echo "<table border='1'>";
    echo "<tr><td>EnrollmentID</td><td>Grade</td><td>EnrollmentSemester</td> 
    <td>CourseID</td><td>StudentID</td><td>Title</td><td>Credits</td> 
    <td>LastName</td><td>FirstMidName</td></tr>";
    while ($row = mysqli_fetch_assoc($result)){
        echo "<tr><td>{$row['EnrollmentID']}</td><td>{$row['Grade']}</td> 
        <td>{$row['EnrollmentSemester']}</td><td>{$row['CourseID']}</td><td> 
        {$row['StudentID']}</td><td>{$row['Title']}</td><td>{$row['Credits']} 
        </td><td>{$row['LastName']}</td><td>{$row['FirstMidName']}</td></tr>";
    }
    echo "</table>";
}

在这个片段中，我们使用 if 语句来检查变量 $_POST['submit'] 存在。
在这里 submit with是表单的提交按钮的名称
注：a $_POST 变量只在表单提交时存在，调用 isset() 会回来的 false 如果表单没有提交，因此你的代码在里面 { } 不会在第一次加载时执行，但会在提交时运行。

您需要查看是否存在“something”（在本例中，您所拥有的只是提交），然后执行显示。。。。

<?php
if($_POST['submit']){
    $host = "localhost";
    $db = "cis475";
    $user = "root";
    $pw = "";

    $conn = new mysqli ($host, $user, $pw, $db);
    if($conn->connect_error) die($conn->connect_error);

    $readAllQuery = "SELECT * FROM enrollment JOIN course ON enrollment.CourseID = course.CourseID JOIN student ON enrollment.StudentID = student.StudentID";

    $result = $conn->query($readAllQuery);
    if (!$result) die($conn->error);

    echo "<table border='1'>";
    echo "<tr><td>EnrollmentID</td><td>Grade</td><td>EnrollmentSemester</td> 
    <td>CourseID</td><td>StudentID</td><td>Title</td><td>Credits</td> 
    <td>LastName</td><td>FirstMidName</td></tr>";
    while ($row = mysqli_fetch_assoc($result)){
        echo "<tr><td>{$row['EnrollmentID']}</td><td>{$row['Grade']}</td> 
        <td>{$row['EnrollmentSemester']}</td><td>{$row['CourseID']}</td><td> 
        {$row['StudentID']}</td><td>{$row['Title']}</td><td>{$row['Credits']} 
        </td><td>{$row['LastName']}</td><td>{$row['FirstMidName']}</td></tr>";
    }
    echo "</table>";
}
?>

<form method='post' action='readAll.php'>

<input type='submit' name='submit' value='Show All Enrollments'>

</form>