(PHP form validation) When I click the submit button of a form, I don't go to another page

This is my first website and I am very new to php (started leaing it two days ago) so please excuse my bad code.

This is what my form looks like:

    <form  action="post.php" method="POST"  id= "form2" onsubmit="retu check();">
                    <label id="fname">
                        First Name <br/>
                        <input type= "text" name= "fiName" id="fn" />
                    </label>
                    <label id="lname">
                        Last Name <br/>
                        <input type= "text" name= "laName" id="ln"/>
                    </label>
                    <label id="phone">
                        Phone Number <br/>
                        <input type= "number" name= "phone" id="ph" />
                    </label>
                    <label id="email">
                        Email Address <br/>
                        <input type= "email" name= "email" id="ea"/>
                    </label>
                    <label id= "pass">
                        Password <br/>
                        <input type = "password" name="pass" id="pas"/>
                    </label>
                    <label id= "conPass">
                        Confirm Password <br/>
                        <input type = "password" name="conPass" id="conPas"/>
                    </label>
                    <input type="submit" value= "Submit" id ="sub"/>
                </form>

My purpose for wanting a new page (post.php) is that I want to check if both password inputs match, that they are at least 8 characters and that no field is left blank. On that new page it will say whether anything is missing or if everything is good to go. Also do you have any tips on how to store that information from different users. (I thought of using fwrite to write into a .txt file, but wouldn't that erase the previous user information?)