Hello,
You need to create a WordFragment and add it as inline content. addStructuredDocumentaTag supports working as an inline WordFragment since the release of phpdocx 13.5.
In the Contents/addStructuredDocumentaTag/sample_1.php example included in the package you can find the following code that illustrates how to add a checkbox and text in the same paragraph:
// checkbox added as WordFragment
$checkboxFragment = new WordFragment($docx, 'document');
$checkboxFragment->addStructuredDocumentTag('checkbox', ['fontSize' => 11, 'checked' => true]);
$docx->addText([array('text' => 'Checkbox: ', 'fontSize' => 12), $checkboxFragment]);
To add the checkbox and then the text you need to set them in the required order:
// checkbox added as WordFragment
$checkboxFragment = new WordFragment($docx, 'document');
$checkboxFragment->addStructuredDocumentTag('checkbox', ['fontSize' => 11, 'checked' => true]);
$docx->addText([$checkboxFragment, array('text' => ' checkbox', 'fontSize' => 12)]);
Regards.