How do I get all the post tags in WordPress?

Use get_tags to get all posts tags

<?php 
$tags = get_tags(array(
  'hide_empty' => false
));
echo '<ul>';
foreach ($tags as $tag) {
  echo '<li>' . $tag->name . '</li>';
}
echo '</ul>';
?>

Try This

$tags = get_tags();
$html = '<div class="post_tags">';
foreach ( $tags as $tag ) {
    $tag_link = get_tag_link( $tag->term_id );

    $html .= "<a href='{$tag_link}' title='{$tag->name} Tag' class='{$tag->slug}'>";
    $html .= "{$tag->name}</a>";
}
$html .= '</div>';
echo $html;