Custom button for WordPress post editor

custom-button.php

<?php 

// Callback function to insert 'styleselect' into the $buttons array
function prefix_mce_buttons( $buttons ) {
	array_unshift( $buttons, 'styleselect' );
	//var_dump($buttons);
	return $buttons;
}
// Register our callback to the appropriate filter
// _2 places the new button on the second line
add_filter('mce_buttons_2', 'prefix_mce_buttons');

// Callback function to filter the MCE settings
function prefix_mce_before_init_insert_formats( $init_array ) {  
	// Define the style_formats array
	$style_formats = array(  
		// Each array child is a format with it's own settings
		array(  
			'title' => 'Button',  
			'selector' => 'a',  
			'classes' => 'button',
			// font awesome must be available in the admin area to see the icon
			'icon'	   => ' fa fa-hand-pointer-o'
		),  
	);  
	// Insert the array, JSON ENCODED, into 'style_formats'
	$init_array['style_formats'] = json_encode( $style_formats );  
	
	return $init_array;  
  
}
// Attach callback to 'tiny_mce_before_init' 
add_filter( 'tiny_mce_before_init', 'prefix_mce_before_init_insert_formats' );