Regex Validate One Letter Multiple Choice - php

Question

New guy here - I have three choices and want to let the user only pass to the next page "4.html" if they select A, else send them to google.com. This is where I've gotten so far :(
if(empty($_POST['choice'])){
echo "Please select at least one choice..!!";
//this should send them to google.com if they select none or the wrong one
}
else{
foreach($_POST['choice'] as $choice){
header('Location: /4.html');
}
}
<form action="multichoice.php" method="post">
<input id="checkbox" type="checkbox" name="choice[]" value="A" />A)Choice A
<input id="checkbox" type="checkbox" name="choice[]" value="B" />B)Choice B
<input id="checkbox" type="checkbox" name="choice[]" value="c" />C)Choice C
<input id="input"onclick="return myFunction()" type="submit" value="Submit"></input>
</form>
I really appreciate your guys' help!

Answer 1

Sidenote: You can't use echo and header together, otherwise you will be outputting before header.
Consult the link following this (footnotes) and is intended to be run inside the same file:
Checkbox method: (which differs from the radio buttons below)
<?php
if(isset($_POST['submit'])){
if(isset($_POST['choice'])){
foreach($_POST['choice'] as $choice){
if($choice == "A"){
echo "You chose A" . "\n";
// header('Location: /4.html');
// exit;
}
if($choice == "B"){
echo "You chose B" . "\n";
}
if($choice == "C"){
echo "You chose C" . "\n";
}
} // brace for foreach
} // brace for if(isset($_POST['choice']))
// else for if(isset($_POST['choice']))
else{
echo "Please make a choice.";
}
} // brace for if(isset($_POST['submit']))
?>
<form action="" method="post">
<input id="checkbox" type="checkbox" name="choice[]" value="A" />A)Choice A
<input id="checkbox" type="checkbox" name="choice[]" value="B" />B)Choice B
<input id="checkbox" type="checkbox" name="choice[]" value="C" />C)Choice C
<input id="input" onclick="return myFunction()" name="submit" type="submit" value="Submit">
</form>
Radio buttons method: (edited) and added a name attribute to the submit button. Sidenote: </input> isn't a valid tag; it's been removed.
<?php
$choice = $_POST['choice'];
if(isset($_POST['submit'])){
if($choice == "A"){
// echo "You chose A" . "\n";
header("Location: /4.html");
exit; // stop further execution
}
if($choice == "B"){
echo "You chose B" . "\n";
}
if($choice == "C"){
echo "You chose C" . "\n";
}
if(empty($_POST['choice'])){
header("Location: http://www.google.com/");
exit; // stop further execution
}
} // submit conditional
?>
<form action="" method="post">
<input id="radio" type="radio" name="choice" value="A" />A)Choice A
<input id="radio" type="radio" name="choice" value="B" />B)Choice B
<input id="radio" type="radio" name="choice" value="C" />C)Choice C
<input id="input" onclick="return myFunction()" name="submit" type="submit" value="Submit">
</form>
Footnotes:
See the following on the subject of outputting before header:
How to fix "Headers already sent" error in PHP
Also, you can use radio buttons for single choices and without the array.
If you're faced with errors/notices/warnings, somewhere down the line:
Add error reporting to the top of your file(s) which will help find errors.
<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);
// rest of your code
Sidenote: Error reporting should only be done in staging, and never production.