search-form.php

<?php
/**
 * Search form.
 *
 * @package   Gamajo
 * @author    Gary Jones
 * @copyright (c) 2015 Gary Jones, Gamajo Tech
 * @link      http://gamajo.com/
 * @license   GPL2-0+
 */

/**
 * Search form class.
 *
 * @package Gamajo
 * @author Gary Jones
 */
class Gamajo_Search_Form {
	protected $unique_id;
	protected $args;

	/**
	 * Merge args and assign to property.
	 *
	 * @since 1.0.0
	 *
	 * @param array $args Optional. Array of args. Default is an empty array.
	 */
	public function __construct( array $args = array() ) {
		$defaults = array(
			/** This filter is documented in genesis/lib/structure/search.php */
			'label'        => apply_filters( 'genesis_search_form_label', 'Search site' ),
			// Placeholder should be empty: http://www.nngroup.com/articles/form-design-placeholders/
			/** This filter is documented in genesis/lib/structure/search.php */
			'placeholder'  => apply_filters( 'genesis_search_text', '' ),
			/** This filter is documented in genesis/lib/structure/search.php */
			'button'       => apply_filters( 'genesis_search_button_text', 'Search' ),
			/**
			 * Filter the ARIA label for the search form button.
			 *
			 * @since 1.0.0
			 *
			 * @param string $button_label Default label is "Search".
			 */
			'button_label' => apply_filters( 'genesis_search_button_label', 'Search' ),
			'url'          => home_url( '/' ),
			'form-class'   => 'search-form',
			'input-name'   => 's',
		);

		$this->args = wp_parse_args( $args, $defaults );
	}

	/**
	 * Get one or all args.
	 *
	 * @since 1.0.0
	 *
	 * @param string $key Optional. Identifier for which string to return. Default is 'all'.
	 *
	 * @return string|array String if key is not 'all', and key exists. Array of all args otherwise.
	 */
	protected function args( $key = 'all' ) {
		if ( 'all' !== $key && isset( $this->args[ $key ] ) ) {
			return $this->args[ $key ];
		}

		return $this->args;
	}

	/**
	 * Get form markup.
	 *
	 * @since 1.0.0
	 *
	 * @return string Form markup.
	 */
	public function get_form() {
		return sprintf(
			'<form method="get" action="%s" role="search" class="%s">%s</form>',
			esc_url( $this->args( 'url' ) ),
			esc_attr( $this->args( 'form-class' ) ),
			$this->get_label() . $this->get_input() . $this->get_button()
		);
	}

	/**
	 * Get label markup.
	 *
	 * @since 1.0.0
	 *
	 * @return string Label markup.
	 */
	protected function get_label() {
		return sprintf(
			'<label for="%s" class="screen-reader-text">%s</label>',
			esc_attr( $this->get_id() ),
			esc_attr( $this->args( 'label' ) )
		);
	}

	/**
	 * Get input markup.
	 *
	 * @since 1.0.0
	 *
	 * @return string Input field markup.
	 */
	protected function get_input() {
		$value = get_search_query() ? apply_filters( 'the_search_query', get_search_query() ) : '';

		// Support Google Custom Search submission
		if ( empty( $value ) && $query = filter_input( INPUT_GET, 'q', FILTER_SANITIZE_STRING ) ) {
			$value = $query;
		}

		return sprintf(
			'<input type="search" name="%s" id="%s" value="%s" placeholder="%s" autocomplete="off" />',
			esc_attr( $this->args( 'input-name' ) ),
			esc_attr( $this->get_id() ),
			esc_attr( $value ),
			esc_attr( $this->args( 'placeholder' ) )
		);
	}

	/**
	 * Get button markup.
	 *
	 * @since 1.0.0
	 *
	 * @return string Button markup.
	 */
	protected function get_button() {
		return sprintf(
			'<button type="submit" aria-label="%s"><span class="search-button-text">%s</span></button>',
			esc_attr( $this->args( 'button_label' ) ),
			$this->args( 'button' )
		);
	}

	/**
	 * Get unique ID.
	 *
	 * @since 1.0.0
	 *
	 * @return string Unique ID.
	 */
	protected function get_id() {
		if ( ! isset( $this->unique_id ) ) {
			$this->unique_id = 'searchform-' . uniqid( '', true );
		}

		return $this->unique_id;
	}
}

Adding an aria-label here is not necessary because the HTML already provides enough information:

<button type="submit" aria-label="%s"><span class="search-button-text">%s</span></button>