functions.php

/**
 * Display search form.
 *
 * @param bool $echo Default to echo and not return the form.
 * @return string|void String when $echo is false.
 */
function astra_get_search_form( $echo = true ) {

	$form = '<form role="search" method="get" class="search-form" action="' . esc_url( home_url( '/' ) ) . '">
		<label>
			<span class="screen-reader-text">' . _x( 'Search for:', 'label', 'astra' ) . '</span>
			<input type="search" class="search-field" ' . apply_filters( 'astra_search_field_toggle_data_attrs', '' ) . ' placeholder="' . esc_attr_x( 'My Placeholder', 'placeholder', 'astra' ) . '" value="' . get_search_query() . '" name="s" role="search" tabindex="-1"/>
		</label>
		<button type="submit" class="search-submit" value="' . esc_attr__( 'Search', 'astra' ) . '"><i class="astra-search-icon"></i></button>
	</form>';
  
  // Replace My Placeholder with your desired string.

	/**
	 * Filters the HTML output of the search form.
	 *
	 * @param string $form The search form HTML output.
	 */
	$result = apply_filters( 'astra_get_search_form', $form );

	if ( null === $result ) {
		$result = $form;
	}

	if ( $echo ) {
		echo $result;
	} else {
		return $result;
	}
}