$3$ David is developing an online education platform that includes a user registration feature.

Users must provide the information as required in Figure $2\mathrm{.}0$ to create an account

Username $($between $5$ to $15$ characters$)$

Password $($at least $8$ characters, including one uppercase letter, one number, and one special character$)$

Email address $($valid email format$)$

Date of birth $($users must be at least $18$ years old$)$

Phone number $(10$ digits, only numbers$)$

Figure $2\mathrm{.}0$

a$)$ Write a complete functional Javascript function to validate the user's information

b$)$ Write a complete functional PHP function to validate the user's information