How do I alter the text of the input field in the search form?

Question

I need to alter the text of the search input field. In my quest to do this, I have dpm() the content of $form on a call to hook_form_search_box_form_alter(). The output is the following.

Array
(
    [search_block_form] => Array
        (
            [#type] => textfield
            [#title] => Search
            [#title_display] => invisible
            [#size] => 15
            [#default_value] => 
            [#attributes] => Array
                (
                    [title] => Enter the terms you wish to search for.
                )

        )

    [actions] => Array
        (
            [#type] => actions
            [submit] => Array
                (
                    [#type] => submit
                    [#value] => Search
                )

        )

    [#submit] => Array
        (
            [0] => search_box_form_submit
        )

    [#form_id] => search_block_form
    [#type] => form
    [#build_id] => form-Y0XlzH4YO-EqVHxg5wnRf-PrnecuUSGdo5LTCPcLgLU
    [form_build_id] => Array
        (
            [#type] => hidden
            [#value] => form-Y0XlzH4YO-EqVHxg5wnRf-PrnecuUSGdo5LTCPcLgLU
            [#id] => form-Y0XlzH4YO-EqVHxg5wnRf-PrnecuUSGdo5LTCPcLgLU
            [#name] => form_build_id
            [#parents] => Array
                (
                    [0] => form_build_id
                )

        )

    [form_id] => Array
        (
            [#type] => hidden
            [#value] => search_block_form
            [#id] => edit-search-block-form
            [#parents] => Array
                (
                    [0] => form_id
                )

        )

    [#id] => search-block-form
    [#method] => post
    [#action] => /
    [#theme_wrappers] => Array
        (
            [0] => form
        )

    [#tree] => 
    [#parents] => Array
        (
        )

    [#validate] => Array
        (
        )

    [#theme] => Array
        (
            [0] => search_block_form
            [1] => search_box
        )

)

Is $form['search_block_form']['#title'] = ''; the right thing to do here?

Answer 1

@NoSssweat has provided a link, it is the answer if you want to alter a placeholder.

But reading your question I am getting other indications also.

FOR D7:

Case-1: If I read the sentence from your question "I need to alter the text of the search input field" I think you are talking about the placeholder, Now what is placeholder- you can see in following image the word "Search the site" in the text-area is called place-holder.

Case-2: But if I read your last sentence, "Is $form['search_block_form']['#title'] = ''; the right thing to do here?" then it talks about other things, this is corrosponds to Text appear when you hover on the input text-area("Enter the terms you wish to search for") as shown in following image.

I will explain both of that, The best tutorial for this is available here: How to Customize the Block Search Form.

Case-1
Paste the following code in your theme's template.php file, change the YourThemeName with your actual theme-name, save & clear the cache.

function YourThemeName_form_search_block_form_alter(&$form, &$form_state, $form_id) {
    $form['search_block_form']['#attributes']['placeholder'] = t('Search the site');
}

Case-2

function YourThemeName_form_search_block_form_alter(&$form, &$form_state, $form_id) {
    $form['search_block_form']['#attributes']['title'] = t('Place a new sentence');
}

Case-3: If you want title on the block, you can just go to block configuration and put the title whatever you want, it will appear as a title of the Block.

FOR D8:

as @NoSssweat pointed out in the following comment, how to do in D8. Hence, I am just repeating that comment here as it is needed.

function YourThemeNAme_form_search_block_form_alter(&$form, FormStateInterface $form_state) {
  $form['keys']['#attributes']['placeholder'] = t('The new placeholder');
}