get_avatar() 是 WordPress 默认获取的 Gravatar 头像的函数，函数通过用户 ID 或邮件地址获取用户头像，同时如果要正确显示头像需要将设置/讨论面板下的“显示头像”给勾选上。该函数的定义位于 wp-includes 文件夹下的 pluggable.php 中，相关代码如下：

if ( !function_exists( 'get_avatar' ) ) :
/**
 * Retrieve the avatar `<img>` tag for a user, email address, MD5 hash, comment, or post.
 *
 * @since 2.5.0
 * @since 4.2.0 Optional `$args` parameter added.
 *
 * @param mixed $id_or_email The Gravatar to retrieve. Accepts a user_id, gravatar md5 hash,
 *                           user email, WP_User object, WP_Post object, or WP_Comment object.
 * @param int    $size       Optional. Height and width of the avatar image file in pixels. Default 96.
 * @param string $default    Optional. URL for the default image or a default type. Accepts '404'
 *                           (return a 404 instead of a default image), 'retro' (8bit), 'monsterid'
 *                           (monster), 'wavatar' (cartoon face), 'indenticon' (the "quilt"),
 *                           'mystery', 'mm', or 'mysteryman' (The Oyster Man), 'blank' (transparent GIF),
 *                           or 'gravatar_default' (the Gravatar logo). Default is the value of the
 *                           'avatar_default' option, with a fallback of 'mystery'.
 * @param string $alt        Optional. Alternative text to use in &lt;img&gt; tag. Default empty.
 * @param array  $args       {
 *     Optional. Extra arguments to retrieve the avatar.
 *
 *     @type int          $height        Display height of the avatar in pixels. Defaults to $size.
 *     @type int          $width         Display width of the avatar in pixels. Defaults to $size.
 *     @type bool         $force_default Whether to always show the default image, never the Gravatar. Default false.
 *     @type string       $rating        What rating to display avatars up to. Accepts 'G', 'PG', 'R', 'X', and are
 *                                       judged in that order. Default is the value of the 'avatar_rating' option.
 *     @type string       $scheme        URL scheme to use. See set_url_scheme() for accepted values.
 *                                       Default null.
 *     @type array|string $class         Array or string of additional classes to add to the &lt;img&gt; element.
 *                                       Default null.
 *     @type bool         $force_display Whether to always show the avatar - ignores the show_avatars option.
 *                                       Default false.
 *     @type string       $extra_attr    HTML attributes to insert in the IMG element. Is not sanitized. Default empty.
 * }
 * @return false|string `<img>` tag for the user's avatar. False on failure.
 */
function get_avatar( $id_or_email, $size = 96, $default = '', $alt = '', $args = null ) {
	$defaults = array(
		// get_avatar_data() args.
		'size'          => 96,
		'height'        => null,
		'width'         => null,
		'default'       => get_option( 'avatar_default', 'mystery' ),
		'force_default' => false,
		'rating'        => get_option( 'avatar_rating' ),
		'scheme'        => null,
		'alt'           => '',
		'class'         => null,
		'force_display' => false,
		'extra_attr'    => '',
	);

	if ( empty( $args ) ) {
		$args = array();
	}

	$args['size']    = (int) $size;
	$args['default'] = $default;
	$args['alt']     = $alt;

	$args = wp_parse_args( $args, $defaults );

	if ( empty( $args['height'] ) ) {
		$args['height'] = $args['size'];
	}
	if ( empty( $args['width'] ) ) {
		$args['width'] = $args['size'];
	}

	if ( is_object( $id_or_email ) && isset( $id_or_email->comment_ID ) ) {
		$id_or_email = get_comment( $id_or_email );
	}

	/**
	 * Filters whether to retrieve the avatar URL early.
	 *
	 * Passing a non-null value will effectively short-circuit get_avatar(), passing
	 * the value through the {@see 'get_avatar'} filter and returning early.
	 *
	 * @since 4.2.0
	 *
	 * @param string $avatar      HTML for the user's avatar. Default null.
	 * @param mixed  $id_or_email The Gravatar to retrieve. Accepts a user_id, gravatar md5 hash,
	 *                            user email, WP_User object, WP_Post object, or WP_Comment object.
	 * @param array  $args        Arguments passed to get_avatar_url(), after processing.
	 */
	$avatar = apply_filters( 'pre_get_avatar', null, $id_or_email, $args );

	if ( ! is_null( $avatar ) ) {
		/** This filter is documented in wp-includes/pluggable.php */
		return apply_filters( 'get_avatar', $avatar, $id_or_email, $args['size'], $args['default'], $args['alt'], $args );
	}

	if ( ! $args['force_display'] && ! get_option( 'show_avatars' ) ) {
		return false;
	}

	$url2x = get_avatar_url( $id_or_email, array_merge( $args, array( 'size' => $args['size'] * 2 ) ) );

	$args = get_avatar_data( $id_or_email, $args );

	$url = $args['url'];

	if ( ! $url || is_wp_error( $url ) ) {
		return false;
	}

	$class = array( 'avatar', 'avatar-' . (int) $args['size'], 'photo' );

	if ( ! $args['found_avatar'] || $args['force_default'] ) {
		$class[] = 'avatar-default';
	}

	if ( $args['class'] ) {
		if ( is_array( $args['class'] ) ) {
			$class = array_merge( $class, $args['class'] );
		} else {
			$class[] = $args['class'];
		}
	}

	$avatar = sprintf(
		"<img alt='%s' src='%s' srcset='%s' class='%s' height='%d' width='%d' %s/>",
		esc_attr( $args['alt'] ),
		esc_url( $url ),
		esc_url( $url2x ) . ' 2x',
		esc_attr( join( ' ', $class ) ),
		(int) $args['height'],
		(int) $args['width'],
		$args['extra_attr']
	);

	/**
	 * Filters the avatar to retrieve.
	 *
	 * @since 2.5.0
	 * @since 4.2.0 The `$args` parameter was added.
	 *
	 * @param string $avatar      &lt;img&gt; tag for the user's avatar.
	 * @param mixed  $id_or_email The Gravatar to retrieve. Accepts a user_id, gravatar md5 hash,
	 *                            user email, WP_User object, WP_Post object, or WP_Comment object.
	 * @param int    $size        Square avatar width and height in pixels to retrieve.
	 * @param string $default     URL for the default image or a default type. Accepts '404', 'retro', 'monsterid',
	 *                            'wavatar', 'indenticon','mystery' (or 'mm', or 'mysteryman'), 'blank', or 'gravatar_default'.
	 *                            Default is the value of the 'avatar_default' option, with a fallback of 'mystery'.
	 * @param string $alt         Alternative text to use in the avatar image tag. Default empty.
	 * @param array  $args        Arguments passed to get_avatar_data(), after processing.
	 */
	return apply_filters( 'get_avatar', $avatar, $id_or_email, $args['size'], $args['default'], $args['alt'], $args );
}
endif;

使用 get_avatar() 函数

对于 get_avatar() 函数的使用我们一般可以通过以下方式调用：

<?php echo get_avatar( $id_or_email, $size, $default, $alt, $args ); ?>

$id_or_email：必填（整型/字符串/对象）。用户 ID 、邮件地址或者评论区评论对象。

$size：选填（整型）。要返回的 Gravatar 头像的大小，最大为 512 默认 96 。

$default：选填（字符串）。自定义的默认头像地址，默认“神秘人物”头像。

$alt：选填（字符串）。img 标签的 alt 属性值，图像的替换文本，默认为空。

$args：选填（数组）。设置新的参数以替换 get_avatar中 的默认参数。

$args 可设置的参数

如果要设置新的参数以替换 get_avatar中 的默认参数，必须在调用 get_avatar 之前声明新的参数，否则它们将不会生效。以下参数均为可选参数，由于使用较少，此处仅简单说下，具体内容请参阅文后官方文档！

size（整型、大小）、height（整型、高度）、width（整型、宽度）、default（字符串、默认头像）、force_default（布尔型、强制使用默认头像）、rating（字符串、头像等级 ‘G’, ‘PG’, ‘R’, ‘X’）、scheme（字符串，设置 URL 的 Scheme）、class（数组或字符串，设置 img 标签的 class 属性）、force_display（布尔型、是否强制显示 Gravatar 头像即使未勾选“显示头像”）、extra_attr（字符串，设置头像的 attributes 属性）。

官方文档：https://codex.wordpress.org/Function_Reference/get_avatar