Introduction to conversion between data types in php learning

The code is as follows:
/*Conversion between data types
* One is forced conversion
* setType(variable, type); //int, integer, float, double, etc.
* This function changes the type of the original variable. Use var_dump(); you can view the variable
*
* Use the (type) form before assignment, which will not change the type of the original variable
* $a=(int)"123abc ";
*
* $variable=intval(variable or value);
* $variable=floatval(variable or value);
* $variable=stringval(variable or value);
*
* Note: integers are in memory Occupies 4 bytes in memory 2.147e9
* Floating point occupies 8 bytes in memory
*
*
* One is automatic conversion (the most common way), the variable will be automatically converted according to the running environment

* and Some common functions related to variables and types
* isset(); // Determine whether a variable exists. If the value is null, it also means empty.
* empty();//Determine whether a variable is empty "",null
* unset();//Delete a variable
* setType();//Set a variable type
* getType();//Get A variable type var_dump(); Get the type and value
*
* Variable type test function
* is_bool();//Judge whether it is a Boolean type
* is_int() is_integer() is_long()//Judge whether it is an integer type
* is_float(), is_double() is_real()//...
* is_array()
* is_object()
* is_resource()
* is_null()
* is_scalar()//Determine whether it is a scalar
* is_numberic()//Determine whether it is any kind of number, or a string of numbers
* is_callable()//Determine whether it is a valid function name

* Declaration and use of constants
* 1. A constant is an identifier of a simple value Symbol
* 2. The value of a constant cannot be changed after it is defined, nor can it be canceled using unset() or other functions
* 3. Constants can be defined and accessed anywhere regardless of the rules of variable scope
* 4 .For constants, use define("constant name", value);
* 5. Do not use "$" when declaring or using constant names
* 6. Constant names are always in uppercase letters
* 7. The value of a constant can only be used Scalar type (int, float, bool, string)
* 8. Constants must be given a value when declared
* 9. defined("constant");//Determine whether the constant exists
*
* Predefined constants and magic Constant
* echo __FILE__;//Output the current file name directory_magic constant
* echo CASE_LOWER;//Output fixed value_predefined constant
*

*/
//This function changes the type of the original variable, use var_dump(); you can view the variable
$str="100.12345abc";
setType($str,int);
var_dump($str);

//Use the (type) form before assignment
$str=" 100.12345abc";
$a=(int)$str;
var_dump($a);//Output int(100)
var_dump($str);//The output value remains unchanged, "100.12345abc"
//If If the string does not start with a number, it is converted to 0

//Different types of operations
$a=10;
$b="100abc";
$c=true;
$d=12.34;
$sum=$a+ $c;//The Boolean type is automatically converted to 1, and the result is 11
$sum=$a+$b;//The result is 110
$sum=$a+$b+$c;//The result is 111
$sum=$ a+$b+$c+$d;//The result is 123.34, because the floating-point memory space is large, and small memory is converted to large memory.

//Determine whether it is an array
$a=array("one","two",1,3,6,8);
if(is_array($a)){
print_r($a);// Print array
}else{
echo $a;
}

//Define constants, use constants
define("home","this is a home");
$a=100;
function demo()
{
global $a;//Because $a is a global variable, it must be called with a global tag
echo $a;
echo home;//Constants can be directly accessed or defined regardless of the scope
}
demo();

/ /Determine whether the constant exists
if(defined("home")
{
echo home;
}
else
{
define("home","this is a home");
}