WP-PageNavi - Bootstrap 4

functions.php

<?php
/**
 * Clean up pagination
 */
add_filter( 'wp_pagenavi', __NAMESPACE__ . '\\gc_pagination', 10, 2 );

function gc_pagination($html) {
    $out = '';
    $out = str_replace('<div','',$html);
    $out = str_replace('class=\'wp-pagenavi\'>','',$out);
    $out = str_replace('<a','<li class="page-item"><a class="page-link"',$out);
    $out = str_replace('</a>','</a></li>',$out);
    $out = str_replace('<span class=\'current\'','<li class="page-item active"><span class="page-link current"',$out);
    $out = str_replace('<span class=\'pages\'','<li class="page-item"><span class="page-link pages"',$out);
    $out = str_replace('<span class=\'extend\'','<li class="page-item"><span class="page-link extend"',$out);  
    $out = str_replace('</span>','</span></li>',$out);
    $out = str_replace('</div>','',$out);
    return '<ul class="pagination">'.$out.'</ul>';
}
?>