以下代码取自"Dynamic image resize"-plugin.

该插件考虑了一个ID和一个字符串。我没有删除这些行，因为它们可能对以后的访问者有用-无论如何，请检查链接。

按ID或URl检索图像

    $hw_string    = image_hwstring( $width, $height );

    $needs_resize = true;

    $error        = false;
    // ID as src
    if ( ! is_string( $src ) )
    {
        $att_id = $src;
        // returns false on failure
        $src    = wp_get_attachment_url( $src );

        // If nothing was found:
        ! $src AND $error = true;
    }
    // Path as src
    else
    {
        $upload_dir = wp_upload_dir();
        $base_url   = $upload_dir[\'baseurl\'];

        // Let\'s see if the image belongs to our uploads directory...
        $img_url = substr(
             $src
            ,0
            ,strlen( $base_url )
        );
        // ...And if not: just return the image HTML string
        if ( $img_url !== $base_url )
        {
            return $this->get_markup(
                 $img_url
                ,$hw_string
                ,$classes
            );
        }

        // Look up the file in the database.
        $file   = str_replace(
             trailingslashit( $base_url )
            ,\'\'
            ,$src
        );
        $att_id = $this->get_attachment( $file );

        // If no attachment record was found:
        ! $att_id AND $error = true;
    }

检查错误，如果未找到附件，则中止。这意味着上传没有按预期结束。

    // Abort if the attachment wasn\'t found
    if ( $error )
    {
        # @TODO Error handling with proper message
        # @TODO Needs a test case
        # remove $file in favor of $error_msg
        /*
        $data = get_plugin_data( __FILE__ );
        $error_msg = "Plugin: {$data[\'Name\']}: Version {$data[\'Version\']}";
        */

        # @TODO In case, we got an ID, but found no image: if ( ! $src ) $file = $att_id;

        return new WP_Error(
             \'no_attachment\'
            ,__( \'Attachment not found.\', \'dyn_textdomain\' )
            ,$file
        );
    }

检查现有图像大小，以防已经有我们需要的大小的图像。如果用户多次尝试上载同一图像，可能会出现这种情况（您必须进行检查以避免重复）。

    // Look through the attachment meta data for an image that fits our size.
    $meta = wp_get_attachment_metadata( $att_id );
    foreach( $meta[\'sizes\'] as $key => $size )
    {
        if (
            $width === $size[\'width\']
            AND $height === $size[\'height\']
            )
        {
            $src = str_replace(
                 basename( $src )
                ,$size[\'file\']
                ,$src
            );
            $needs_resize = false;
            break;
        }
    }

处理调整大小现在我们确定有附件，需要调整大小。我们还将更新post 和attachment post类型元数据。

    // If an image of such size was not found, ...
    if ( $needs_resize )
    {
        $attached_file = get_attached_file( $att_id );
        // ...we can create one.
        $resized       = image_make_intermediate_size(
             $attached_file
            ,$width
            ,$height
            ,true
        );

        if ( ! is_wp_error( $resized ) )
        {
            // Let metadata know about our new size.
            $key = sprintf(
                 \'resized-%dx%d\'
                ,$width
                ,$height
            );
            $meta[\'sizes\'][ $key ] = $resized;
            $src = str_replace(
                 basename( $src )
                ,$resized[\'file\']
                ,$src
            );

            wp_update_attachment_metadata( $att_id, $meta );

            // Record in backup sizes, so everything\'s
            // cleaned up when attachment is deleted.
            $backup_sizes = get_post_meta(
                 $att_id
                ,\'_wp_attachment_backup_sizes\'
                ,true
            );

            ! is_array( $backup_sizes ) AND $backup_sizes = array();

            $backup_sizes[ $key ] = $resized;

            update_post_meta(
                 $att_id
                ,\'_wp_attachment_backup_sizes\'
                ,$backup_sizes
            );
        }

现在一切都应该好了，准备好继续。

如果您不需要所有内容，只需删除您不需要的内容，但确保不遗漏所需的错误检查。安全总比后悔好。