我正在创建一个元盒来上载PDF文件。

对于这一行代码，我有以下警告通知：$this_file = $filearray[\'url\'];

警告：中的字符串偏移量“url”非法

完整的功能代码：

function wp_custom_attachment() {
    wp_nonce_field( plugin_basename(__FILE__), \'wp_custom_attachment_nonce\' );
    $html = \'<p class="description">Upload your PDF here.</p>\';
    $html .= \'<input id="wp_custom_attachment" name="wp_custom_attachment" size="25" type="file" value="" />\';

    $filearray = get_post_meta( get_the_ID(), \'wp_custom_attachment\', true );
    $this_file = $filearray[\'url\'];
    
    if ( $this_file != \'\' ) { 
         $html .= \'<div><p>Current file: \' . $this_file . \'</p></div>\'; 
    }
    echo $html; 
}

编辑

function add_custom_meta_boxes() {
    add_meta_box( 
        \'wp_custom_attachment\',
        \'PDF File\',
        \'wp_custom_attachment\',
        \'workshop\',
        \'normal\'
    ) ;
}
add_action( \'add_meta_boxes\', \'add_custom_meta_boxes\' );

function wp_custom_attachment() {
    wp_nonce_field( plugin_basename(__FILE__), \'wp_custom_attachment_nonce\' );
    $html = \'<p class="description">Upload your PDF here.</p>\';
    $html .= \'<input id="wp_custom_attachment" name="wp_custom_attachment" size="25" type="file" value="" />\';

    $filearray = get_post_meta( get_the_ID(), \'wp_custom_attachment\', true );
    $this_file = $filearray[\'url\'];
    
    if ( $this_file != \'\' ) { 
         $html .= \'<div><p>Current file: \' . $this_file . \'</p></div>\'; 
    }
    echo $html; 
}

function save_custom_meta_data( $id ) {
    if ( ! empty( $_FILES[\'wp_custom_attachment\'][\'name\'] ) ) {
        $supported_types = array( \'application/pdf\' );
        $arr_file_type = wp_check_filetype( basename( $_FILES[\'wp_custom_attachment\'][\'name\'] ) );
        $uploaded_type = $arr_file_type[\'type\'];

        if ( in_array( $uploaded_type, $supported_types ) ) {
            $upload = wp_upload_bits($_FILES[\'wp_custom_attachment\'][\'name\'], null, file_get_contents($_FILES[\'wp_custom_attachment\'][\'tmp_name\']));
            if ( isset( $upload[\'error\'] ) && $upload[\'error\'] != 0 ) {
                wp_die( \'There was an error uploading your file. The error is: \' . $upload[\'error\'] );
            } else {
                add_post_meta( $id, \'wp_custom_attachment\', $upload );
                update_post_meta( $id, \'wp_custom_attachment\', $upload );
            }
        }
        else {
            wp_die( "The file type that you\'ve uploaded is not a PDF." );
        }
    }
}
add_action( \'save_post\', \'save_custom_meta_data\' );

function update_edit_form() {
    echo \' enctype="multipart/form-data"\';
}
add_action( \'post_edit_form_tag\', \'update_edit_form\' );