Use of variables in php learning_PHP tutorial

//Quote
$one="test";
two=&$one;//Equivalent to passing an address, two variables point to one address

//Dynamic variable
$one="######";
$ two="one";
$three="two";

echo $three."
";//Output "two"
echo $$three."< br>";//Output "one"
echo $$$three."
";//Output "######"

//There are 8 in php Type
//4 types of scalars: int integer
// bool boolean
// float, double, real
// string
//2 types of composite types: array
/ / object
//2 special types: resource type resource
// empty type null


//declaration of integer
$int=10; //declaration of decimal
$int=045;//Octal declaration
$int=0xff;//Hexadecimal declaration
$float=3.14E+5;//Scientific notation
$float=3.14E -5;

//All cases are false
$bool=false;
$bool=0;
$bool=0.000;
$bool=null;
$bool="";
$bool=" ";
$bool="0";
$bool=array();

//String declaration
//1. Both single quotes and double quotes can declare strings
//2. There is no length limit for the declared strings
//3. In double-quoted strings, variables can be parsed directly , and you can use escape characters directly (you can escape the single quote itself, or you can escape the escape character "")
//4. In a single-quoted string, variables cannot be parsed directly, nor can Use escape characters
//5. Double quotes can no longer be used within double quotes, and single quotes can no longer be used within single quotes
//6. It is best to use single quotes,
$str=' aaaaa';
$str="aaaa";
//Delimiter declares a string, a large number of strings
//test is a custom string, there cannot be any characters after it, and there are no spaces. No
//It must also end with the custom string test, and there cannot be any characters before the end
$str=<<this write content...
test;
?>