It's a normal case for researchers to have some manuscripts under reviewing. So, can such only-submitted and currently-unpublished papers add some value to CV of whom apply for a PhD program? Should they be included in CV at all?
The rule of thumb with writing a CV is to keep things relevant. If what you are writing is relevant for the purpose you are writing your CV (you should really have a template CV, and customize it for each application), then yes add them.
So a couple of ways how I could see that being relevant for you, specifically:
you mention applying to a PhD program, so it might be relevant to show whoever is taking the decision that you have worked on something similar before
in a similar way, it could be meaningful to show that you have some research experience, as opposed to other graduates that have 0 research experience.
if your manuscript in review has been uploaded to a preprint server (like arXiv or BioRxiv) then you might even be able to showcase the work you have done.
Unpublished texts are not only OK, but sometimes essential. When considering applicants for a PhD position I want to have samples of the applicants' scientific writings. In the absence of published articles, these have to be unpublished texts, or even internship reports. (Reports are sometimes better than articles, because articles can have several authors, while a report is typically written by the applicant alone.)
So the answer is: yes, list all your relevant writings, published or not. Give links to the full texts that are available online, otherwise be prepared to send them on request.