- substr: documentation ( source)
- strpos: documentation ( source)
<?php
$r = '<div class="form-group>';
$label = '<label class="control-label" for="id_f-variants-99999-is_active">Is active</label>';
$label = substr($r, 0, strpos($r, ">"));
$label .= '>';
echo($label);
echo($r);