- 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 .= '>';
$r .= $label;
$r .= '<input id="id_f-variants-99999-is_active" class="form-control checkbox" type="checkbox" name="f-variants-99999-is_active"></input>';
$r .= '<span class="unchecked">hoa</span>';
$r .= '<span class="checked">hoahoahoa</span>';
$r .= '</label>';
$r .= '</div>';
echo($r);