这种方式对我很有效，无需设置$GLOBAL:

function xxx_wp_email_content_type() {
    return \'text/html\';
}

function xxx_wp_retrieve_password_message( $content, $key ) {
    add_filter( \'wp_mail_content_type\', \'xxx_wp_email_content_type\' );

    ob_start();

    $email_subject = xxx_wp_retrieve_password_title();

    include( \'templates/email_header.php\' );
    include( \'templates/lost_password_email.php\' );
    include( \'templates/email_footer.php\' );

    $message = ob_get_contents();
    ob_end_clean();

    return $message;
}
add_filter( \'retrieve_password_message\', \'xxx_wp_retrieve_password_message\', 10, 2 );

从技术上来说，这并不能回答这个问题，但更多的是问题背后的问题。您可能希望仅为密码重置电子邮件设置内容类型的原因是，将所有系统电子邮件设置为text/html 将破坏默认密码重置电子邮件。

我建议使用retrieve_password_message 过滤器与wp_mail_content_type 筛选以使密码重置电子邮件与HTML格式兼容：

<?php
// adding support for html emails
// this converts ALL wp_mail emails to HTML, which messes up the password reset
add_filter( \'wp_mail_content_type\',\'squarecandy_set_content_type\' );
function squarecandy_set_content_type() {
        return "text/html";
}

// add this filter too
// this will make the password reset email compatible with the HTML format
add_filter( \'retrieve_password_message\', \'squarecandy_retrieve_password_message\', 10, 1 );
function squarecandy_retrieve_password_message( $message ) {
    // Revise the message content to make it HTML email compatible
    $message = str_replace(\'<\',\'\',$message);
    $message = str_replace(\'>\',\'\',$message);
    $message = str_replace("\\n",\'<br>\',$message);
    // make any additional modifications to the message here...
    return $message;
}