How can the misuse of PHP functions like isHTML lead to issues with email content formatting and delivery in PHPMailer?

The misuse of PHP functions like isHTML can lead to issues with email content formatting and delivery in PHPMailer because it may incorrectly detect the content type of the email, leading to unexpected behavior or errors. To solve this issue, it is important to properly set the isHTML property in PHPMailer to ensure that the content type is correctly identified.

// Set the isHTML property to true if the email content is HTML formatted
$mail->isHTML(true);