Integrate Mailgun in my PHP website

I have a website built with core PHP. I am using shared hosting using CPanel.(Composer not installed) When I send an email to my customers the email is delivered into to spam instead of inbox. I send sales payment confirmation email to my customers. I want all my emails to be delivered in the inbox only so my customers can get notification easily. So I have decided to use mailgun. Mailgun is used for sending transactional emails. I want you to to integrate mailgun in PHP code so I can just send emails to my customers in their inbox. The will set some variables in a template and then send email to the user. It will be a rich text template. Make a PHP function which will take the recipient email ID and variables as parameters. This function will set the parameters in the template and then send the email to the recipient. I am not using any PHP framework. It's a core PHP website. Will send only transactional emails.