PHP: NULL - Manual

User Contributed Notes
NULL

disappear at dissolution dot ath dot cx
02-May-2005 06:20


Hi,



Im using PHP 5.0.3



i wrote a small null study to test the cases here and this is the results i got



Code ::



<?php



    $Array = array ( 0 , '' , FALSE , NULL ) ;

    $ArrayCount = count ( $Array ) ;



    $String .= '$Array = ' . "array ( 0 , '' , FALSE , NULL ) <br><br>" ;



    for ( $i = 0 ; $i < $ArrayCount ; $i++ )



    {



        if ( $Array [ $i ] == NULL )



        {



            $String .= '$Array [ $i ] == NULL :: $Array [ ' . $i . ' ] <br>' ;



        }



        if ( $Array [ $i ] === NULL )



        {



            $String .= '$Array [ $i ] === NULL :: $Array [ ' . $i . ' ] <br>' ;



        }



        if ( is_null ( $Array [ $i ] ) )



        {



            $String .= 'is_null ( $Array [ $i ] ) :: $Array [ ' . $i . ' ] <br>' ;



        }



    }



    echo $String ;



?>



Results ::



$Array = array ( 0 , '' , FALSE , NULL )



$Array [ $i ] == NULL :: $Array [ 0 ]

$Array [ $i ] == NULL :: $Array [ 1 ]

$Array [ $i ] == NULL :: $Array [ 2 ]

$Array [ $i ] == NULL :: $Array [ 3 ]

$Array [ $i ] === NULL :: $Array [ 3 ]

is_null ( $Array [ $i ] ) :: $Array [ 3 ]

jaumesb aat consert doot net
26-Feb-2005 03:38


Note that the expression 



( $v == NULL )



will evaluate as TRUE if $v is zero or the empty string.



To avoid this, remember to use : 



( $v === NULL )

rizwan_nawaz786 at hotmail dot com
19-Oct-2004 12:22


Hi

 Rizwan Here

   

   Null is the Constant in PHP. it is use to assign a empty value to the variable like



  $a=NULL;



  At this time $a has is NULL or $a has no value;



  When we declaire a veriable in other languages than that veriable has some value depending on the value of memory location at which it is pointed but in php when we declaire a veriable than php assign a NULL to a veriable.

pozmu at wp dot pl
20-Apr-2004 11:21


responding to joemamacow at hotmail dot com comment: 

if you are using isset() to check is the variable set, the more logically and clear way to "delete" the variable is to use unset() (http://www.php.net/unset ) function:



<?php



unset($variable);



?>



Of course setiing variable value to NULL is also OK.

alex at netflex dot nl
13-Jun-2002 07:13


Hi,



Function for looking if it is a NULL



<?php



  $var = NULL;



  if (isnull("var")) {

    echo "var===NULL\n";

  } else {

    echo "var!==NULL\n";

  }



  if (isnull("test")) { // give FALSE, test is not set

    echo "test===NULL\n";

  } else {

    echo "test!==NULL\n";

  }



  $array['var'] = NULL;



  if (isnull("var", $array)) {

    echo "array['var']===NULL\n";

  } else {

    echo "array['var']!==NULL\n";

  }



  function isnull($var, $base = FALSE) {

    if ($base===FALSE) {

      $base = &$GLOBALS;

    } elseif (!is_array($base)) {

      return FALSE;

    }

    if ((array_key_exists($var, $base))&&($base[$var]===NULL)) {

      return TRUE;

    } else {

      return FALSE;

    }

  }



?>

avbentem at hetnet.nl
22-Feb-2002 04:25


To extend a bit on tbdavis's comment:



:: NULL == NULL is true

:: NULL == FALSE is true

:: NULL == TRUE is false



However: note the implicit type conversions that PHP performs! When using 'identical' instead of 'equal' then both NULL === FALSE and NULL === TRUE yield FALSE. An overview is easily created using something like



    function evalExpr( $desc )

    {

      echo str_pad($desc , 15) . "--> ";

      var_dump( eval( "return(" . $desc . ");" ));

    }



Note that even TRUE AND TRUE does not evaluate to a boolean value, and that OR and XOR behave different as well!



    PHP Version: 4.0.6



    false          --> bool(false)

    true           --> bool(true)

    null           --> NULL

    !null          --> bool(true)



    true and true  --> int(1)

    true and false --> int(0)

    true or true   --> int(1)

    true or false  --> int(1)

    true xor true  --> bool(false)

    true xor false --> bool(true)

 

    true == null   --> bool(false)

    true === null  --> bool(false)

    true != null   --> bool(true)

    true !== null  --> bool(true)



    false == null  --> bool(true)

    false === null --> bool(false)

    false != null  --> bool(false)

    false !== null --> bool(true)



    null == null   --> bool(true)

    null != null   --> bool(false)

    null === null  --> bool(true)

    null !== null  --> bool(false)



    null or null   --> int(0)

    null xor null  --> bool(false)

    null and null  --> int(0)

  

    true or null   --> int(1)

    true xor null  --> bool(true)

    true and null  --> int(0)



    false or null  --> int(0)

    false xor null --> bool(false)

    false and null --> int(0)



    true < null    --> bool(false)

    true > null    --> bool(true)

    false < null   --> bool(false)

    false > null   --> bool(false)



    1 + null       --> int(1)

    "text" . null  --> string(4) "text"



Finally, for those who do not know SQL: in SQL the NULL value is evaluated a bit like "I do not know; it could be anything, like 0, 1, a, b, true, false or even nothing at all". This implies that in SQL NULL == NULL could be interpreted as "could be anything" == "could be something else", which does not yield true! Instead, it yields NULL, which boils down to FALSE in boolean context...



Likewise, in SQL:



    NULL AND TRUE yields NULL

    NULL OR TRUE yields TRUE

    NULL AND FALSE yields FALSE

    NULL OR FALSE yields NULL

    NULL == TRUE yields FALSE

    NULL == FALSE yields FALSE



a.

sc at dbtech dot de
16-Feb-2002 06:59


NULL as the best way to detect additional parameters of unknown type:



function FooBar($Param = NULL) {

  if (is_null($Param)) {

  [...]

junk dot phpnet at gasolinemm dot com
18-Nov-2001 12:19


$a = "";


$b = NULL;





$a == $b; 


/* returns true: $a has been converted to $b for the equality comparison */





is_null($a); //returns false


is_null($b); //returns true





$a === $b;


/* returns false: $a is not _identical_ to $b */

dward at maidencreek dot com
12-Nov-2001 06:52


Nulls are almost the same as unset variables and it is hard to tell the difference without creating errors from the interpreter:





$var = NULL;





isset($var) is FALSE


empty($var) is TRUE


is_null($var) is TRUE





isset($novar) is FALSE


empty($novar) is TRUE


is_null($novar) gives an Undefined variable error





$var IS in the symbol table (from get_defined_vars())


$var CAN be used as an argument or an expression.





So, in most cases I found that we needed to use !isset($var) intead of is_null($var) and then set $var = NULL if the variable needs to be used later to guarantee that $var is a valid variable with a NULL value instead of being undefined.

tbdavis at greyshirt dot net
11-Oct-2001 07:36


Unlike the relational model, NULL in PHP has the following properties:


NULL == NULL is true,


NULL == FALSE is true.


And in line with the relational model, NULL == TRUE fails.

NULL

Syntax