PHP: WordPress Custom Meta Box

add_meta_box.php

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

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

function new_meta_box() {
	global $post;

	$custom_fields = get_post_custom( $post->ID );
	$input         = $custom_fields['input'][0];
	$textarea      = $custom_fields['textarea'][0];
	?>
	
	<p><strong>Input</strong></p>
	<p>
	<label class="screen-reader-text" for="input">Input</label>
	<input style="width:97%;" id="input" name="input" value="<?php echo $input; ?>" />
	</p>

	<p><strong>Text Area</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 ( ! isset( $_POST['input'] ) ) return;
	if ( ! ( get_post_type( $post_id ) == 'post_type' ) ) return;
	
	update_post_meta( $post_id, 'input', $_POST['input'] );
	update_post_meta( $post_id, 'textarea', $_POST['textarea'] );
}