WordPress: Conditional Text Area Meta Box

text-area-meta.php

// custom meta box
add_action( 'admin_menu', 'create_meta_box' );
add_action( 'save_post', 'save_meta_box' );

function create_meta_box() {

$post_id = $_GET['post'] ? $_GET['post'] : $_POST['post_ID'] ;
$template_file = get_post_meta($post_id,'_wp_page_template',TRUE);
  // check for a template type
  if ($template_file == 'homepage.php') {


	// add_meta_box( $id, $title, $callback, $post_type, $context, $priority, $callback_args );
	add_meta_box( 'new_meta_box', 'Meta Box Title', 'new_meta_box', 'page', 'side', 'low' );
}

}

function new_meta_box() {
	global $post;

	$custom_fields = get_post_custom( $post->ID );

	$textarea      = $custom_fields['textarea'][0];
	?>

	<p><strong>Enter text</strong></p>
	<p>
	<label class="screen-reader-text" for="textarea">Text Area</label>
	<textarea style="width:100%;" id="textarea" name="textarea" rows="6"><?php echo $textarea; ?></textarea>
	</p>
	<?php
}

function save_meta_box($post_id) {
	global $post;


	if ( ! ( get_post_type( $post_id ) == 'page' ) ) return;


	update_post_meta( $post_id, 'textarea', $_POST['textarea'] );
}