variables are case sensitive
function names are case-insensitive
class names are case-insensitive
key words are case-insensitive
constructs are case-insensitive

**********
8 data types: 
int 
float-point
string
Booleans

arrays
objects

resource(handle)
NULL(case insensitive)

****************
evaluate to false:

false
0
0.0
"0"
""
an array with 0 elements
an object with no values or functions
NULL

*****************



*****************

A variable that has not been set value behaves like NULL

******************

variables defined outside a function(called global variables) is not accessible inside a function.

variable scope: 
local
global
static
function

*****************
php garbage collection :
reference counting
copy-on-write


********************

== same 
=== identical, datatype also matches


********************
Logical oeprator 
&& and
|| or
xor
!

Bitwise operator
~
%
|
^
<<
>>
********************

(int)
(float)
(array)
(object)
(string)
(bool)
*********************
In Switch/While/For,
you can specify an optional number for break/continue keyword to break/continue out of.

*********************

require a nonexistent file is a fatal error, while include will give a warning and moves on. 
*********************

silence warning:
@include
get_included_files()
**********************
<?= : Direct echo.
No space between ? and = 

<input type="text" value="<?="ds";?>"/>
only works with shot_open_tag

<input type="text" value="<?php="ds";?>"/>
not working.


**********************
Functions only compiled once for the page.
**********************

<?php
	function cols(){ ?>
</td><td> //here no php //codes just HTML tags
<?php } ?>

**********************
The inner function cannot be called until the outer function is called.



function paramters can never be global variables.

a static variable only initialized the first time the function is called.

a parameter that is assigned as passed by ref can only be a variable. 

when using passing by value, PHP MUST copy the value.

**********************

A function can have any number of param that have default values, but they must listed behind the parameters that do not default values.

*****************

func_num_args()
func_get_args()
func_get_arg()
******************

The result of a function with variable parameters cannot be directly used as a parameter for another function.
f_A(function_with_var_parameter());// Error!



*******************

function with & beore its name returns an alias(reference) to its return value.


*******************

() variable-function
function_exists()

*******************

language constructs such as 'echo', 'isset' cannot be called from a variable function