Question : What to insert into PHP form to help prevent hacking

I am new to PHP.
I am using Apache for Windows and PHP 5.
I have set up a Contact form and I want to make it safer.
Below is the file.
I have read where it is easy to hack because of carriage return, etc.
What and where should I insert code to help prevent this.

We will use the PHP to send our form results through an email.
<?php
//--------------------------Set these paramaters--------------------------

// Subject of email sent to you.
$subject = 'Results from Contact form';

// Your email address. This is where the form information will be sent.
$emailadd = '[email protected]';

// Where to redirect after form is processed.
$url = 'http://www.zzz.com';

// Makes all fields required. If set to '1' no field can not be empty. If set to '0' any or all fields can be empty.
$req = '0';

// --------------------------Do not edit below this line--------------------------
$text = "Results from form:\n\n";
$space = ' ';
$line = '
';
foreach ($_POST as $key => $value)
{
if ($req == '1')
{
if ($value == '')
{echo "$key is empty";die;}
}
$j = strlen($key);
if ($j >= 20)
{echo "Name of form element $key cannot be longer than 20 characters";die;}
$j = 20 - $j;
for ($i = 1; $i <= $j; $i++)
{$space .= ' ';}
$value = str_replace('\n', "$line", $value);
$conc = "{$key}:$space{$value}$line";
$text .= $conc;
$space = ' ';
}
mail($emailadd, $subject, $text, 'From: '.$emailadd.'');
echo '<META HTTP-EQUIV=Refresh CONTENT="0; URL='.$url.'">';
?>

Thanks
Bob

Answer : What to insert into PHP form to help prevent hacking

use captcha to help protecting from spamming