Most journals labeled "open-access" are "open-access only". You don't have any other option for these. Because they are open-access, the journal can't make money in more traditional ways like selling access to university libraries - who would pay to access content that's already open? (I'm aware I'm simplifying a bit here) Be wary that predatory journals also fit in this category, so while there is nothing wrong in principle with open access you do want to make sure you're only paying for publication in legitimate journals.
Other journals use a more traditional model, but also offer an open-access option, often in part to comply with funding agency requirements that require authors to publish with some level of open-access. These journals will give you a choice when you submit, but make sure you are complying with the requirements of any funding you've received. Also note that many journals (including reputable ones) require some page charges for authors whether they are open-access or not; open-access costs more, but non-open-access isn't necessarily free.
If you want to publish under a different model than a journal offers, you need to find a different journal.
Two other things to think about: 1) PhD students shouldn't ever be paying out of their own pocket to publish; funds that pay for students to do research are often also available to pay publishing costs. Talk to your advisor. 2) Some journals have some sort of discount for financial hardship, which you should be aware of in case you can qualify.