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<?php 
function theme_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'search_block_form') {
    $form['search_block_form']['#title'] = t('Search'); // Change the text on the label element
    $form['search_block_form']['#title_display'] = 'invisible'; // Toggle label visibilty
    //$form['search_block_form']['#size'] = 40;  // define size of the textfield
    $form['search_block_form']['#default_value'] = t('Search'); // Set a default value for the textfield
    //$form['actions']['submit']['#value'] = t('GO!'); // Change the text on the submit button
    $form['actions']['submit'] = array('#type' => 'image_button', '#src' => base_path() . path_to_theme() . '/immagini/btnCerca.png');

    // Add extra attributes to the text box
    $form['search_block_form']['#attributes']['onblur'] = "if (this.value == '') {this.value = 'Cerca';}";
    $form['search_block_form']['#attributes']['onfocus'] = "if (this.value == 'Cerca') {this.value = '';}";
  }
}