adding function to Class

Hi,

I’m learning to build a plugin with Classes and I’m stuck trying to take a function as place it inside my class and call it from there.

The ‘sanitize_callback‘ for my ‘erynpro_global_css‘ setting calls an external function. I want to place that function inside my class and call it from there.

Can someone help me achieve this.

Thanks
Steven

My Class so far

<?php
/**
 * Plugin Name: *************
 * Plugin URI: **********
 * Description: *************
 * Version: ***********
 * Author: *****************
 * Author URI: *************
 * Text Domain: proeryn
 */

if ( ! defined( 'ABSPATH' ) ) {
	die();
}

class Eryn_plugin {

    protected $plugin_slug;
    private static $instance;

    public static function get_instance() {
        if( null == self::$instance ) {
                self::$instance = new Eryn_plugin();
        }
        return self::$instance;
    } 

    private function __construct() {

            add_action( 'customize_register', array( $this, 'eryn_plugin_register_theme_customizer' ));
            add_action( 'wp_head', array( $this, 'eryn_plugin_customizer_css' ));
    } 

    public function eryn_plugin_register_theme_customizer( $wp_customize ){

        //Section
        $wp_customize->add_section( 'erynpro_custom_css' , array(
            'title'         => __('Custom CSS', 'erynpro'),
            'description'   => 'Add your custom CSS which will overwrite the theme CSS',
            'panel'         => 'eryn_header_panel',
            'priority'      => 999,
        ) );

        // Setting
        $wp_customize->add_setting('erynpro_global_css', array(
            'sanitize_callback'	=> 'erynpro_sanitize_textarea',
        ));

        // Control
        $wp_customize->add_control(
            new Customize_CustomCss_Control(
                $wp_customize,
                'custom_css',
                array(
                    'label'      => __('Custom CSS', 'erynpro'),
                    'section'    => 'erynpro_custom_css',
                    'settings'   => 'erynpro_global_css',
                    'type'		 => 'custom_css',
                    'priority'	 => 9999
                )
            )
        );
    }

    public function eryn_plugin_customizer_css() {
        ?>
            <style id="pro_eryn_css">
                <?php if(get_theme_mod( 'erynpro_global_css' )) : ?>
                    <?php echo get_theme_mod( 'erynpro_global_css' ); ?>
                <?php endif; ?>
            </style>
        <?php
    }

}
add_action( 'after_setup_theme', array( 'Eryn_plugin', 'get_instance' ) );

function Eryn_plugin_css_customize_register($wp_customize) {

    class Customize_CustomCss_Control extends WP_Customize_Control {
        public $type = 'custom_css';

        public function render_content() {
            ?>
            <label>
            <span class="customize-control-title"><?php echo esc_html( $this->label ); ?></span>
            <textarea style="width:100%; height:150px;" <?php $this->link(); ?>><?php echo $this->value(); ?></textarea>
            </label>
            <?php
        }
    }
}
add_action( 'customize_register', 'Eryn_plugin_css_customize_register' );

function erynpro_sanitize_textarea($input) {
    global $allowedposttags;
    $output = wp_kses( $input, $allowedposttags);
    return $output;
}