Slack team invite sign-up form handler

slack.php

<?php
/*
Slack team invite sign-up form handler
Henry Stark 2015
Original Idea: https://levels.io/slack-typeform-auto-invite-sign-ups/
*/

/* VARIABLES TO EDIT */
$slackTeam = 'scanlation';// Slack team name
$slackToken = 'xoxp-1234';// Slack API token (Generate at https://api.slack.com/web)
$slackJoinChannels = 'C0D2ET69W';// Separate IDs with a comma

$usedEmailsFile = $_SERVER["DOCUMENT_ROOT"].'/sho/prevEmails.json';// File for used emails
$formName = 'name';// Form name field name var
$formEmail = 'email';// Form email field name var
/* END VARIABLES */

mb_internal_encoding("UTF-8");

if(isset($_POST[$formName]) && isset($_POST[$formEmail])){
    $name = htmlspecialchars($_POST[$formName]);
    $email = htmlspecialchars($_POST[$formEmail]);

    // Load used emails file
    if(@!file_get_contents($usedEmailsFile)) {
        $usedEmails = array();
    } else {
        $usedEmails = json_decode(file_get_contents($usedEmailsFile),true);
    }
    $offset = count($usedEmails);

    if(!in_array($email, $usedEmails)) {// If account has not been registered via form
        $slackInviteUrl = 'https://'.$slackTeam.'.slack.com/api/users.admin.invite?t='.time();
		// URL fields
        $fields = array(
            'email' => urlencode($email),
            'channels' => urlencode($slackJoinChannels),
            'first_name' => urlencode($name),
            'token' => $slackToken,
            'set_active' => urlencode('true'),
            '_attempts' => '1'
        );
		// Convert to url
        $fields_string='';
        foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
        rtrim($fields_string, '&');

		// POST connection to Slack API
        $ch = curl_init();
        curl_setopt($ch,CURLOPT_URL, $slackInviteUrl);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch,CURLOPT_POST, count($fields));
        curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);

		$replyRaw = curl_exec($ch);
        $reply = json_decode($replyRaw,true);
        if($reply['ok'] == false) {// Success
            echo('Error regitstering: '.$reply['error']);
        }
        else {// Error posting
			echo('Thank you for your signing up, '.$name.'!. You have been added to the <a href="//'.$slackTeam.'.slack.com">'.$slackTeam.'</a> Slack team.<br/>Check your email ('.$email.') for an invite link.');
        }
		curl_close($ch);

        // Save email in used emails file
        array_push($usedEmails, $email);
		file_put_contents($usedEmailsFile, json_encode($usedEmails));

    } else {// Error email in use
        echo('Error: The email '.$email.' has already been used to sign up in the team <a href="//'.$slackTeam.'">'.$slackTeam.'</a>.');
	}
} else {// Error name or email values not set
    echo('Error: Missing data. Please (re)submit the form.');
}

Thank you gael! I'll change this soon