Booleans: What is the difference between NULL and empty?

[eldan88]

Hi,

 

I  am pretty sure this questions gets asked a lot. I would like to know what is the difference between those two. I know they share similarities. But from what I know a 0 integer or a 0 string is considered empty.  How do I know when to use NULL and When to use empty ? Thanks!

[rythemton]

It all depends on what you are doing:

$a = "";

$b = 0;

$c = NULL;

$d = "0";

 

function            $a        $b        $c        $d

is_numeric          false    true      false    true

is_null            false    false    true      false

is_string          true      false    false    true

empty              true      true      true      true

 

I've used it for multipurpose functions before:

function multiPurpose( $value=NULL ) {
  if( is_null( $value ) ) {
    // this means no value was sent
  }
  elseif( is_numeric( $value ) ) {
    // this means a number was sent
  }
  elseif( is_array( $value ) ) {
    // this means an array was sent
  }
  else {
    // if it got here, it is none of the above, probably text
  }
}

And sometimes it doesn't really matter.

[mr.noname]

in my opinion

 

the NULL is not assign anything to the variable

and

most of default value are EMPTY value

so if check it by empty() it return TRUE

but if check by is_null()  it should be return FALSE

 

$a = NULL; // i think $a will not keep anything
$b;  // it assign to the default value --> i think it must be the empty value

[rythemton]

$a = NULL; // i think $a will not keep anything
$b;  // it assign to the default value --> i think it must be the empty value

Actually, both is_null( $a ) and is_null( $b ) will return TRUE responses. An unset or uninitialized variable is considered to have a NULL value. Setting a value to be NULL will also return a FALSE with the isset function, even though the value is set. Let's expand the chart:

$a = "";

$b = 0;

$c = NULL;

$d = "0";

unset( $e );

 

function            $a        $b        $c        $d        $e

is_numeric          false    true      false    true      false

is_null            false    false    true      false    true

is_string          true      false    false    true      false

empty              true      true      true      true      true

isset              true      true      false    true      false

==""                true      true      true      false    true

===""              true      false    false    false    false

[chrsm]

Hi,

 

I  am pretty sure this questions gets asked a lot. I would like to know what is the difference between those two. I know they share similarities. But from what I know a 0 integer or a 0 string is considered empty.  How do I know when to use NULL and When to use empty ? Thanks!

 

I would recommend reading the PHP manual, on the "type comparison" page: http://www.php.net/manual/en/types.comparisons.php

 

in my opinion

 

the NULL is not assign anything to the variable

and

most of default value are EMPTY value

so if check it by empty() it return TRUE

but if check by is_null()  it should be return FALSE

 

$a = NULL; // i think $a will not keep anything
$b;  // it assign to the default value --> i think it must be the empty value

 

$a 'exists' but is set to NULL; isset and is_null will return FALSE and TRUE, respectively.

 

NULL is not really a value, but the variable exists. For example:

$ php -r 'if(is_null($b)) { echo("\$b is null\n"); }'
PHP Notice:  Undefined variable: b in Command line code on line 1

Notice: Undefined variable: b in Command line code on line 1
$b is null

 

$b, in this case, was undefined, and something that isn't set is "NULL" in a way, but since it isn't set, it's going to trigger an error. If $b had be assigned NULL in a previous expression, there would be no warning triggered.

 

Furthermore, there is no default value for a variable, so one cannot define a variable like so:

$b;

Without assigning it to anything. It will trigger an error if $b is referenced by an expression.

 

$ php -r '$b; echo($b);'
PHP Notice:  Undefined variable: b in Command line code on line 1

Notice: Undefined variable: b in Command line code on line 1

 

Anyways, the tables on the page I linked have a very good explanation of how these things all map to each other.

 

Cheers.

[mr.noname]

Thank so much  chrsm and rythemton

 

is it not have the default value  because php has a loosely typed language

 

 

[rythemton]

I thought I'd check to make sure what I was saying was accurate and created a test script. Here are my results: