What specific PHP.ini settings could be causing issues with sending HTML emails using PHP mail() function?

The issue with sending HTML emails using the PHP mail() function could be related to the PHP.ini settings for the "sendmail_path" and "mail.add_x_header" directives. To solve this issue, ensure that the "sendmail_path" is correctly set to the path of the sendmail program on your server and set "mail.add_x_header" to "0" to prevent additional headers from being added to the email.

// Set the sendmail path in PHP.ini
ini_set("sendmail_path", "/usr/sbin/sendmail -t -i");

// Disable additional headers in emails
ini_set("mail.add_x_header", "0");