描述：

该函数用于从主题文件夹中导入searchform.php文件，无参数。

用法：

<?php get_search_form( $echo ); ?>

参数：

$echo

(boolean) (可选)；‘true’ 输出搜索表单;‘false’ 将表单以字符串形式返回.

默认值: true

返回值：

(string)

如果$echo参数设置为false，则为表单的html。

源文件：

/**
 * Display search form.
 *
 * Will first attempt to locate the searchform.php file in either the child or
 * the parent, then load it. If it doesn't exist, then the default search form
 * will be displayed. The default search form is HTML, which will be displayed.
 * There is a filter applied to the search form HTML in order to edit or replace
 * it. The filter is 'get_search_form'.
 *
 * This function is primarily used by themes which want to hardcode the search
 * form into the sidebar and also by the search widget in WordPress.
 *
 * There is also an action that is called whenever the function is run called,
 * 'pre_get_search_form'. This can be useful for outputting JavaScript that the
 * search relies on or various formatting that applies to the beginning of the
 * search. To give a few examples of what it can be used for.
 *
 * @since 2.7.0
 *
 * @param bool $echo Default to echo and not return the form.
 * @return string|void String when $echo is false.
 */
function get_search_form( $echo = true ) {
	/**
	 * Fires before the search form is retrieved, at the start of get_search_form().
	 *
	 * @since 2.7.0 as 'get_search_form' action.
	 * @since 3.6.0
	 *
	 * @link https://core.trac.wordpress.org/ticket/19321
	 */
	do_action( 'pre_get_search_form' );
 
	$format = current_theme_supports( 'html5', 'search-form' ) ? 'html5' : 'xhtml';
 
	/**
	 * Filter the HTML format of the search form.
	 *
	 * @since 3.6.0
	 *
	 * @param string $format The type of markup to use in the search form.
	 *                       Accepts 'html5', 'xhtml'.
	 */
	$format = apply_filters( 'search_form_format', $format );
 
	$search_form_template = locate_template( 'searchform.php' );
	if ( '' != $search_form_template ) {
		ob_start();
		require( $search_form_template );
		$form = ob_get_clean();
	} else {
		if ( 'html5' == $format ) {
			$form = '<form role="search" method="get" class="search-form" action="' . esc_url( home_url( '/' ) ) . '">
				<label>
					<span class="screen-reader-text">' . _x( 'Search for:', 'label' ) . '</span>
					<input type="search" class="search-field" placeholder="' . esc_attr_x( 'Search …', 'placeholder' ) . '" value="' . get_search_query() . '" name="s" title="' . esc_attr_x( 'Search for:', 'label' ) . '">
				</label>
				<input type="submit" class="search-submit" value="'. esc_attr_x( 'Search', 'submit button' ) .'">
			</form>';
		} else {
			$form = '<form role="search" method="get" id="searchform" class="searchform" action="' . esc_url( home_url( '/' ) ) . '">
				<div>
					<label class="screen-reader-text" for="s">' . _x( 'Search for:', 'label' ) . '</label>
					<input type="text" value="' . get_search_query() . '" name="s" id="s">
					<input type="submit" id="searchsubmit" value="'. esc_attr_x( 'Search', 'submit button' ) .'">
				</div>
			</form>';
		}
	}
 
	/**
	 * Filter the HTML output of the search form.
	 *
	 * @since 2.7.0
	 *
	 * @param string $form The search form HTML output.
	 */
	$result = apply_filters( 'get_search_form', $form );
 
	if ( null === $result )
		$result = $form;
 
	if ( $echo )
		echo $result;
	else
		return $result;
}