Basic record of getting started with PHP

Basics ：
1. Format
    <?php echo "hello!"; ?>
2.ceho : Output
3. Connector :.
4. At the end of the paper ;
5. With notes ：//

To configure ：
1.apche -> httpd-vhosts.conf  ( Maybe there's a version problem )
2.www Catalog modification [menu.left]
    .ini     
    .tpl
  The problem of directory garbled ： use notepad++ Open to change the encoding to :ANSI
3. Multisite configuration
4. Port settings ：apche-httpd.conf ,2 A place to ：
    1.listen : 80
    2.serverName localhost : 80
    
Variable
1. $
$var = " Hello ";
echo $var;
2. function var_dump, Display the data type of the variable  
    $name = " Zhang San ";echo $name; //string(6)
    $name = "123"; echo $name;//string(3)
    $n = 10;echo $n;//int(10)
3. Variable names must start with a letter or underscore
  Variable names can only be made of letters 、 Numbers 、 as well as “_” form , It can also contain Chinese characters
  Variable names cannot contain spaces . When the variable name consists of multiple words , It is recommended to use “_” separation （ such as $my_apple）, Commonly known as underline method , Or start with a capital letter, for example $myApple, Commonly known as camel nomenclature （ Also known as hump nomenclature ）.
  One thing in particular to note , stay PHP Variable names in are case sensitive , Such as “$my_book” And “$my_Book” Represents two different variables .
4.memory_get_usage Get current PHP Memory consumed .
5. Boolean type （boolean）： There are only two values , One is TRUE, the other one FALSE, It can be understood as yes or no . It's case insensitive
  When we use ”echo” When an instruction outputs a boolean type , If it is “true” The output is “1”,“false” Then nothing is output
6. String type can be defined in three ways ： Single quotation marks 、 Double quotation marks and Heredoc Structure form .
7. When double quotes contain variables , Variables will be concatenated with the contents in double quotation marks ;
  When single quotation marks contain variables , Variables are output as strings .
8.Heredoc Structural form method to solve this problem , First, use the delimiter to represent the string （<<<）, And then “<<<“ Then provide an identifier GOD, Then there's the string , Finally, end the string with the supplied identifier
  $string1 = <<<wacao
  I have a little donkey , I never ride .
  One day I had a whim , Ride to the market .
  I have a small whip in my hand , I am proud of myself .d
  I don't know why , I fell all over with mud .
  wacao;
9.NULL（NULL）：NULL Is an empty type , Case insensitive ,NULL Type has only one value , Indicates that a variable has no value , When assigned to NULL, Or not yet assigned , Or be unset(), In these three cases, the variable is considered to be NULL.
    unset($var3);// Release $var3  
    var_dump($var3); NULL

Constant
1. Defined constant , It uses PHP The function in define() Definition
  define("PI",3.14);// Define constants
  echo PI;//3.14
2. The system constant is PHP Already defined constants , We can use it directly , Common system constants are ：
（1）__FILE__ :php Program file name . It can help us get the physical location of the current file on the server .
（2）__LINE__ :PHP Number of lines of program file . It can tell us , What line is the current code in .
（3）PHP_VERSION: The version number of the current parser . It can tell us the current PHP The version number of the parser , We can know our... In advance PHP Whether the code can be PHP Parser parsing .
（4）PHP_OS： Execute the current PHP Version of the operating system name . It can tell us the name of the operating system used by the server , We can optimize our code according to the operating system
3. Use constant() function . It has the same effect as using constant name output directly
  $p="";
  $area = constant($p)*$r*$r;
4.defined() Function can help us determine whether a constant has been defined
  
5. Operator
  And Logic and
  Or Logic or
  Xor Logical XOR
  Not Logic is not