$string = str_replace('target="_blank"', ' ', $string);
$string = str_replace("target=\"_blank\"", " ", $string);
$string = str_replace('target=\"_blank\"', ' ' , $string);
<?php
$string = '<!-- html vode -->';
$pattern = '/target=["|\']_blank["|\']/g';
$replacement = ' ';
echo preg_replace($pattern, $replacement, $string);
?>