currently I am trying to get the PHP mail() function to work. For this, I would probably need to enable httpd_can_sendmail, but when I try to change it on rhc ssh, permission is denied, even if I try to change it to non-persistently. Access to the sudoers file is also denied, and I feel like I just can't find the right words to find the solution on Google.
How can I set my account to be able to change those booleans?
If this is for OpenShift Online, then you do not have access to modify SELinux at all. You will also not have sudo or root access. You only have access to what is inside of your container.
If you are just trying to send email with PHP, you should use a third party like SendGrid (available at https://marketplace.openshift.com as an easy to add service), or mailgun, or even your own authenticated gmail/yahoo email account. That will also help ensure that your emails don't get caught in the OpenShift spam filter.