UTM Variable A/B Test Content Shortcode

abtest-shortcode.php

<?php

function ab_tester_shortcode_callback($atts, $content = null){
  $display = false;
  $a = shortcode_atts(array(
    'variant_key' => 'utm_variant',
    'variant' => 'default',
    'method' => 'GET'
  ),$atts);

  if($a['method']=='GET'){
    if( ( $_GET[$a['variant_key']]==$a['variant'] ) ||
        ( $a['variant']=='default' && ( empty( $_GET[$a['variant_key']] ) ) )
      )
    {
      $display = true;
    }
  }

  if($display==true){
    echo $content;
  }
}

add_shortcode('ab_tester_content','ab_tester_shortcode_callback');

Need to rewrite to allow a "default" as well in case of missing GET variable.