Update Select Statement

UPDATE subject_section
SET facultyid =
(SELECT facultyid FROM ref_subject
WHERE subjectid = subject_section.subjectid) 

PHP array function

Check value exist in array

in_array($value_to_check, $array_to_check);

Check key exist in array

array_key_exists($key_to_check, $array_to_check);

SQL count the same value in selected field

Count number of user with same DOB in DB 

SELECT dob,
COUNT(dob) AS NumOccurrences
FROM members
GROUP BY dob
HAVING ( COUNT(dob) > 0 )