- var_dump: documentation ( source)
- implode: documentation ( source)
- explode: documentation ( source)
- array_diff: documentation ( source)
<?php
$separator = ' ';
$old_string = 'abc def';
$new_tag = 'def';
$tags = explode($separator, $old_string);
$diffTags = array_diff($tags, array($new_tag));
var_dump(implode($separator, $diffTags) . $separator . $new_tag);
var_dump($tags == $diffTags);