Variable issues in PHP

#$abc = "abc";
$def;
#var_dump(isset($abc));
//var_dump(empty($def));
//var_dump(false);
//var_dump(empty (''));
var_dump(isset($def));
var_dump(is_null($def));
#var_dump(isset($def));
#1.php data variables are similar to C/C++ Declaration and definition of variables in?
        #There are no declarations in php, only undefined and defined
    #2. What is the definition of php variables? Is it $def; or $abc = "abc"; ?
# The definition of a variable in php is to directly assign a value to the variable when writing the variable, that is, $abc="abc"; This is called variable definition
# If $def, What will be displayed? Let’s do an experiment var_dump($def);
#Output:
#Notice: Undefined variable: def in C:UserslsqbuptDesktopabcdefg.php on line 9
#NULL
# $def; This is called a variable being undefined

#Let’s do it again Do an experiment
  # If $def = null; var_dump($def);
  # It will output NULL and there will be no Notice
 
  #
  #3.isset()   is_null()   empty() and the variable itself as When judging if($abc) or if($def), is there any difference between these functions?
#4.What is null? Is it case sensitive?
       #NULL empty type
   #null is not case-sensitive, the NULL type has only one value, indicating that a variable has no value, the variable is considered NULL in the following three situations #1. Assigned to NULL
    #2. Not yet assigned a value
        #3. Being unset();
      The function of the #is_null() function can still be seen literally, it is quite simple
  #isset() and is_null() are a pair
  #isset() function: when { Undefined, such as $def} and {the variable itself is NULL, such as $def = null;}, it returns false and there is no notice. Others return true
#empty() function: when '',"",0,'0 ' ,"0" ,null ,false ,array(), undefined variables are returned as true
#is_null() function, when {undefined, such as $def} and {the variable itself is NULL, such as $def = null ;} returns true and others returns false
                                                      #is_null() function will have a notice for {undefined, such as $def}, but there will be no notice for {the variable itself is NULL, such as $def = null;} {I found this function It's a pair with isset()}

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The above has introduced the variable issues in PHP, including aspects of it. I hope it will be helpful to friends who are interested in PHP tutorials.