Submit goes to action page even without value - php

Question

Every time I press submit. It goes straight to the (action) page without checking to see if any value is filled in.How do I stop it from changing pages without first checking to see if any information is filed in the name email and comment?
<?php
if( empty($name) && isset($_POST["submit"])){
global $connect;
$name = $_POST['name'];
$email = $_POST['email'];
$comment = $_POST["comment"];
$sql = "INSERT INTO entries (fullName,email,comment)
VALUES ('$name', '$email', '$comment')";
$result = mysqli_query($connect, $sql);
if(!$result){
die("try again" .mysqli_error());
}
}
?>
<section>
<div class="container text-center ">
<h1>Guess Book</h1>
<div class="row"></div>
<form class="text-center" action="thankYou.php" method="post" id="signUp" >
<label class="mb-2 mt-2" style="padding-right:1vh" for="name">Please Enter Your Full Name Here:</label><input type="text" name="name" placeholder="Enter Full Name" minlength="3" ><br>
<label class="mb-2 mt-2" style="padding-right:4.1vh" for="email">Please Enter Your Email Here:</label><input type="email" name="email" placeholder="Enter Email" minlength="4" ><br>
<div class="form-group ">
<label for="comment">Comment:</label>
<textarea name="comment" class="form-control mb-3" rows="8" minlength="4" id="comment"></textarea>
</div>
<input class="bottom-pad" type="submit" name="submit">
</form>
</div>
</section>

Answer 1

the simplest option is to use html "required" attribute
in your case:
<input type="text" name="name" placeholder="Enter Full Name" minlength="3" required>
this is client side validation, you also always need to validate server side.
your code:
if( empty($name) && isset($_POST["submit"])){
is slightly wrong, here is the fixed version
if( !empty($_POST['name']) && isset($_POST["submit"])){

Answer 2

The reason why it get submited , it because your not doing any validation in the form for a minimal validation you can use required on the input field
Please consider using a better validation i just used this for the purpose of demonstration
<section>
<div class="container text-center ">
<h1>Guess Book</h1>
<div class="row"></div>
<form class="text-center" action="thankYou.php" method="post" id="signUp" >
<label class="mb-2 mt-2" style="padding-right:1vh" for="name">Please Enter Your Full Name Here:</label><input type="text" name="name" placeholder="Enter Full Name" minlength="3" required><br>
<label class="mb-2 mt-2" style="padding-right:4.1vh" for="email">Please Enter Your Email Here:</label><input type="email" name="email" placeholder="Enter Email" minlength="4" required><br>
<div class="form-group ">
<label for="comment">Comment:</label>
<textarea name="comment" class="form-control mb-3" rows="8" minlength="4" id="comment" required></textarea>
</div>
<input class="bottom-pad" type="submit" name="submit">
</form>
</div>
</section>

Answer 3

You can do form validation like the previous answers have suggested. This will prevent the form from submitting unless the inputs that are required have a value in them.
You should also get in the habit of validating your code on the server side as well to make sure the data submitted by the user is the information you expect to be submitted by the user.
Here is some simple code to check to make sure that the values at least have something in them. You can take it further to validate that each field has data that meets certain criteria like a name only having alpha characters.
if($_POST["submit"] && isset($_POST["submit"])){
global $connect;
$name = $_POST['name'];
$email = $_POST['email'];
$comment = $_POST["comment"];
if(!empty($_POST["name"]) && !empty($_POST["email"]) && !empty($_POST["comment"])){
$sql = "INSERT INTO entries (fullName,email,comment)
VALUES ('$name', '$email', '$comment')";
$result = mysqli_query($connect, $sql);
if(!$result){
die("try again" .mysqli_error());
}
} else {
echo 'You must enter enter a value for all fields.';
}
}