Need help with PHP: Show an image field when video field is empty
Trust on Views empty field settings :-)

Hide the image and the test field. Configure the video field empty setting telling that should show the image field: use the right token, something like this:

[field_image]


Then configure the image field empty setting with the text field.

So, if there's no vídeo Views will try to show the image field. When rednering the image field Vies will show the image if it's present, or the text field if image is also empty.