Can not send mail from html form

I have been working on a single page website. It have like 8 pages. Last one is contact form. I have a PHP function "send_mail.php" that posts the data to my gmail account. This code is only working if i put it in a separate html but if i call this function through my index.html, it doesnt send the email.

<section id="contact">
    

        

            

                

                    

                        
Contact Info
                        
                            

                                
                            
                            

                                
                            
                            

                                
                            
                            

                                
                            
                            <input type="submit" value="Submit" />
                        </form>
                    </div>
                </div>
            </div>
        </div>
    </div>
</section><!--/#bottom-->

and then my php

    <?php function isInjected($str) {
    $injections = array('(\n+)',
    '(\r+)',
    '(\t+)',
    '(%0A+)',
    '(%0D+)',
    '(%08+)',
    '(%09+)'
    );
    $inject = join('|', $injections);
    $inject = "/$inject/i";
    if(preg_match($inject,$str)) {
        return true;
    }
    else {
        return false;
    }}$email_address = $_REQUEST['email_address'] ; 
$name = $_REQUEST['user_name'] ;
$subject = $_REQUEST['email_subject'] ;
$comments = $_REQUEST['comments'] ;

// If the user tries to access this script directly, redirect them to feedback form,
if (!isset($_REQUEST['email_address'])) {
header( "Location: index.html" );
}

// If the form fields are empty, redirect to the error page.
elseif (empty($email_address) || empty($comments)) {
header( "Location: index.html" );
}

// If email injection is detected, redirect to the error page.
elseif ( isInjected($email_address) ) {
header( "Location: index.html" );
}

// If we passed all previous tests, send the email!
else {
mail( "abcd@gmail.com", $subject,
  $comments, "From: $email_address" );
header( "Location: index.html" );
}
?>

Somehow I found the error location but couldnt narrow it down. If I remove my js references, i can send email though.