function yourtheme_preprocess_search_block_form(&$vars, $hook) {
	// Modify elements of the search form
	unset($vars['form']['search_block_form']['#title']);

	// Set a default value for the search box
  $vars['form']['search_block_form']['#value'] = t('Search this site');
	
	// Add a custom class to the search box
	// Set yourtheme.css > #search-block-form .form-text { color: #888888; }
  $vars['form']['search_block_form']['#attributes'] = array(
  	 'onblur' => "if (this.value == '') {this.value = '".$vars['form']['search_block_form']['#value']."';} this.style.color = '#000000';", 
		 'onfocus' => "if (this.value == '".$vars['form']['search_block_form']['#value']."') {this.value = '';} this.style.color = '#000000';" );

	// Modify elements of the submit button
	unset($vars['form']['submit']);

	// Change text on the submit button
	//$vars['form']['submit']['#value'] = t('Go!');

	// Change submit button into image button - NOTE: '#src' leading '/' automatically added
	$vars['form']['submit']['image_button'] = array('#type' => 'image_button', '#src' => 'sites/all/themes/yourtheme/images/search-button.png');

	// Rebuild the rendered version (search form only, rest remains unchanged)
  unset($vars['form']['search_block_form']['#printed']);
  $vars['search']['search_block_form'] = drupal_render($vars['form']['search_block_form']);

	// Rebuild the rendered version (submit button, rest remains unchanged)
	unset($vars['form']['submit']['#printed']);
	$vars['search']['submit'] = drupal_render($vars['form']['submit']);
	
	// Collect all form elements to print entire form
  $vars['search_form'] = implode($vars['search']);
}

http://drupal.org/node/154137

Customize Block Search Form in Drupal 6

template, drupal

PHP