Disable submit button if input field is empty - php

Question

**Hello..i know my type of question has been answered in different questions before;but i tried them all..none worked!So please have a look on my issue.
I've a table that contains form input fields where values come from database.I didn't wanted the values to be edited.So used "readonly". But the problem is:By the inspect element of a browser when readonly is removed..then the value can be edited and blank input can be submitted !!! So i want to disable the editing or at least want to disable the submit button if input field is empty.**
The code of the table:
<?php
if (isset($_POST['show'])) {
$class = $_POST["Class"];
$sql = "SELECT * FROM students WHERE Class='$class' ORDER BY Roll ";
$result = $conn->query($sql);
if ($result->num_rows > 0) {
// output data of each row
?>
<form action="" method="POST">
<table class="table table-bordered table-hover" style="width: 85%;text-align: center">
<tr >
<th>Roll</th>
<th>Student's Name</th>
<th>Attendance</th>
</tr>
<?php while ($row = $result->fetch_assoc()) { ?>
<tr>
<td><input value="<?php echo $row['Roll']; ?>" name="Roll[]" readonly required=""/></td>
<td><input value="<?php echo $row['Name']; ?>" name="Name[]" readonly required=""/></td>
<td><select name="Status[]">
<option value="0">0</option>
<option value="1">1</option>
</select></td>
</tr>
<?php } ?>
</table>
<input type="submit" name="save" value="Save" style="width: 50%;margin-left: 20%">
</form>
<?php
} else {
$message = "Sorry! No result!";
echo "<script type='text/javascript'>alert('$message');</script>";
}
$conn->close();
}
?>
The insertion code:
<?PHP
if (isset($_POST["save"])) {
foreach ($_POST["Roll"] as $rec => $value) {
$Roll = $_POST["Roll"][$rec];
$Name = $_POST["Name"][$rec];
$Status = $_POST["Status"][$rec];
$Date = date('Y-m-d');
$sql = "INSERT INTO `attendance`(`id`, `Date`, `Roll`, `Name`, `Status`) VALUES ('','$Date','$Roll','$Name','$Status')";
}
if ($conn->query($sql) === TRUE) {
$message = "Saved !";
echo "<script type='text/javascript'>alert('$message');</script>";
} else {
echo "Error: " . $sql . "<br>" . $conn->error;
}
}
?>

Answer 1

this is correct way to not input empty field
$class = $_POST["Class"];
if(!empty($class)) {
$sql = "SELECT * FROM students WHERE Class='$class' ORDER BY DESC or ASC";
$result = $conn->query($sql);
if ($result->num_rows > 0) {
// output data of each row
}
}
?>

Answer 2

I edited part of your code to disallow editing. I hope it serves your pourpose. i used disabled attribute on the input tags.
<tr>
<td><input value="<?php echo $row['Roll']; ?>" name="Roll[]" disabled/></td>
<td><input value="<?php echo $row['Name']; ?>" name="Name[]" disabled/></td>
<td><select name="Status[]">
<option value="0">0</option>
<option value="1">1</option>
</select></td>
</tr>