我试着用transition_post_status. 我想在里面设置一些条件语句。为了看看是否一切正常，我想放置一个调试语句。问题是，一旦我设置了echo语句，我就会得到输出以及两条警告消息：

Error

Warning: Cannot modify header information - headers already sent by (output started at /Applications/MAMP/htdocs/sandbox/wp-content/plugins/media-meta-require/media-meta-require.php:193) in /Applications/MAMP/htdocs/sandbox/wp-admin/post.php on line 233

Warning: Cannot modify header information - headers already sent by (output started at /Applications/MAMP/htdocs/sandbox/wp-content/plugins/media-meta-require/media-meta-require.php:193) in /Applications/MAMP/htdocs/sandbox/wp-includes/pluggable.php on line 1178

无论我如何调用错误字符串，错误都会出现。使用普通回显字符串：

function mmr_attachment_check( $new_status, $old_status, $post ) {
    if( $new_status === \'publish\' ) {
        echo \'Error\';
    }
}
add_action( \'transition_post_status\', \'mmr_attachment_check\', 10, 3 );

使用包含html元素的回显：

function mmr_attachment_check( $new_status, $old_status, $post ) {
    if( $new_status === \'publish\' ) {
        echo \'<div class="error"><p>Error</p></div>\';
    }
}
add_action( \'transition_post_status\', \'mmr_attachment_check\', 10, 3 );

或者如果调用外部函数：

function mmr_attachment_check( $new_status, $old_status, $post ) {
    if( $new_status === \'publish\' ) {
        mmr_admin_notice();
    }
}
add_action( \'transition_post_status\', \'mmr_attachment_check\', 10, 3 );

function mmr_admin_notice() {
    global $pagenow;
    if( $pagenow == \'post.php\') {
        echo \'<div class="error"><p>Error</p></div>\';
    }
}

只要我注释掉与echo相关的行，它就会毫无错误地通过。我还参考了wp故障排除文档problem. 但在我的例子中，我找不到任何可能导致这种行为的空格或类似物/

add_action( \'transition_post_status\', function ( $new_status, $old_status, $post ) {
    if( $new_status === \'publish\' ) {
        add_action( \'admin_head\', function () {
            echo \'To test if we get this message without headers already sent error message\';
        }, PHP_INT_MAX );
    }, 10, 3 );