Function to check if input field is empty - php

Question

I'm trying to validate a form field.
The function validate_emp checks if the input field is empty. I want to know why it's not working.
$first_name = $middle_name = $last_name = $gender = $email = "";
$first_nameErr = $middle_nameErr = $last_nameErr = $genderErr = $emailErr="";
function validate_emp($input_name, $error_mess, $field_name){
if(empty($_POST[$field_name])){
$error_mess = "This field is must not empty.";
}else{
$input_name = $_POST[$field_name];
}
}
if(isset($_POST["btnregister"])){
validate_emp($first_name, $first_nameErr, "first_name");
validate_emp($middle_name, $middle_nameErr, "middle_name");
validate_emp($last_name, $last_nameErr, "last_name");
validate_emp($gender, $genderErr, "gender");
validate_emp($email, $emailErr, "email");
}
This is the form that I've created
<form method="POST">
First Name: <input type="text" name="first_name" placeholder="First Name" value="<?php echo $first_name ?>"> <span class="error"> <?php echo $first_nameErr ?> </span><br>
Middle Name: <input type="text" name="middle_name" placeholder="Middle Name" value="<?php echo $middle_name ?>"> <span class="error"> <?php echo $middle_nameErr ?> </span><br>
Last Name: <input type="text" name="last_name" placeholder="Last Name" value="<?php echo $last_name ?>"> <span class="error"> <?php echo $last_nameErr ?> </span><br>
<select name="gender">
<option name="gender" value="">Gender</option>
<option name="gender" <?php if($gender == "Male") { echo "Selected"; }?> value="Male">Male</option>
<option name="gender" <?php if($gender == "Female") { echo "Selected"; }?> value="Female">Female</option>
</select> <span class="error"> <?php if ($gender == "") {echo $genderErr;} ?> </span> <br>
Email: <input type="text" name="email" value="<?php echo $email ?>"><span class="error"> <?php echo $emailErr ?> </span><br>
<input type="submit" name="btnregister" value="Register">
</form>

Answer 1

You haven't returned anything from the function try returning something.
function validate_emp($input_name, $error_mess, $field_name){
if(empty($_POST[$field_name])){
$error_mess = "This field is must not empty.";
}else{
$input_name = $_POST[$field_name];
}
return //
}

Answer 2

I think the problem is that when you do:
validate_emp($first_name, $first_nameErr, "first_name");
validate_emp($middle_name, $middle_nameErr, "middle_name");
validate_emp($last_name, $last_nameErr, "last_name");
validate_emp($gender, $genderErr, "gender");
validate_emp($email, $emailErr, "email");
the first and second parameters of the function are using variables that aren't defined.
I can see you are using them as variables by reference, and in order to do so you must use them with & before them, if I where you I would return empty or the value and evaluate that:
function validate_emp($field_name){
if(empty($_POST[$field_name])){
$output = "";
}else{
$output = $_POST[$field_name];
}
return $output;
}
Then outside of the function:
$first_name = validate_emp("first_name");
if ($first_name == "") { echo "$first_name cannot be empty"; }