how can i stop spam form submissions from bots?

<?php // RAY_form_token.php
 
// LOCAL FUNCTIONS TO HANDLE FORM TOKENS
// CREATE AN IDENTITY IN THE FORM
function make_form_token() {
	$string	= "CHANGE THIS IF YOU WANT" . time() . $_SERVER["SCRIPT_FILENAME"] . "?";
	$token 	= md5($string);
	$_SESSION["_form_token"]	= $token;
return $token;
}
 
// EVALUATE THE IDENTITY IN THE FORM
function check_form_token($token=null) {
	if (empty($token)) { $token = $_POST["_form_token"]; }
	if ($token == $_SESSION["_form_token"]) {
		$_SESSION["_form_token"] = md5($_SESSION["_form_token"]); // MUNG THE TOKEN
return true;
	}
return false;
}
 
// MODIFY THIS IF YOU WANT A FRIENDLY FORM TOKEN ERROR
function form_token_error() {
	die("Server Error F");
}
 
// SESSION IS REQUIRED
session_start();
 
// CHECK FOR FORM INPUT
if (!empty($_POST)) {
 
// SHOW THE FORM TOKEN
	$token = $_SESSION["_form_token"];
	echo "<br />The form token is $token ";
	if (check_form_token()) {
		echo "and it is valid.\n";
	} else {
		echo "but it is NOT valid.\n";
	}
echo "<br />Refresh this screen to resend the data and you can  see a form token error.\n";
}
 
?>
<br /><br />
Click GO to see the form token.
<form action="<?=$PHP_SELF?>" method="post">
<input type="hidden" name="_form_token" value="<?=make_form_token()?>" />
<input type="submit" name="submit" value="Go!" />
</form>

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