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<?php
$our_parsed_url = parse_url( site_url() );
$link_parsed_url = parse_url( get_sub_field( 'link' ) );

if ( $our_parsed_url['host'] === $link_parsed_url['host'] ) :
	$external_link = false;
else :
	$external_link = true;
endif;
?>
<a class="button" <?php if($external_link): echo'target="_blank"'; endif;?> href="<?php the_sub_field( 'link' ); ?>">Visit</a>