Add one or more classes to the WordPress search form's 'Search' button

modify-search.php

<?php

/**
 * Add one or more classes to the WordPress search form's 'Search' button
 * @author Alain Schlesser (alain.schlesser@gmail.com)
 * @link http://www.brightnucleus.com/add-class-wordpress-search-button/
 *
 * @param  string  $form   the search form HTML output
 * @return string          modified version of the search form HTML output
 *
 * @see http://codex.wordpress.org/Function_Reference/get_search_form
 * @see http://developer.wordpress.org/reference/hooks/get_search_form/
 */
function as_adapt_search_form( $form ) {
    // $forms contains the rendered HTML output of the standard search form
    // the exact output is changed if your theme has declared HTML5 support
    // with the following minimum settings:
    //
    // add_theme_support( 'html5', array( 'search-form' ) );
    //
    // see http://codex.wordpress.org/Function_Reference/add_theme_support

    // add Foundation classes to the button class
    //
    // we do a string replace and search for 'search-submit', which is the one
    // class that is already defined for the HTML5 search form
    //
    // if HTML5 search-form support has not been defined, this will leave the
    // HTML output unchanged
    $form = str_replace(
        'search-submit',
        'search-submit small radius button',
        $form
    );

    // return the modified string
    return $form;
}

// run the search form HTML output through the newly defined filter
add_filter( 'get_search_form', 'as_adapt_search_form' );

?>

See article at the Bright Nucleus Homepage:

• Add a Class to the WordPress Search Button