How to send email with SMTP in php

Question

I need to send an email using SMTP. Previously I was using PHP mail() But now my client wants me to use SMTP.

Here is my code:

$payer_email = "Your Email";
$subject = "Your Subject";
$message = 'Dear '.$name.',
            Thank you for your purchase from '.$site_url.'. The details of your purchase are below.
            Transaction ID: '.$txn_id.'
            Item Name: '.$item_name.'
            Payment Amount: '.$payment_amount.'
            Payment Amount: '.$payment_status.'
            Paid to: '.$receiver_email.'
            Thanks and Enjoy!';

$headers .= 'From: ' .$from. "\r\n" .'Reply-To: ' .$from . "\r\n";
$headers  .= 'MIME-Version: 1.0' . "\r\n";
$headers  .= "Content-Type: text/html; charset=iso-8859-1 ";

//mail to buyer
mail( $payer_email , $subject, $message, $headers );

Can someone please tell me any ideas or share any tutorial to do this?