Search Google/Youtube to understand the functions clearly.
- echo/print/printf
- json_encode & json_decode
- isset
- in_array
- empty
- count
- date
- explode
- implode
- trim/ltrim/rtrim
- str_replace
- strlen
- str_replace
- substr / strpos
- date/time
- unset
- is_array
- die()
- phpinfo
- array_filter
- array_unique
