isset vs empty vs is_null

PHP has multiple functions to check PHP variables with respect to their initialized values. These functions are isset, empty and is_null.

• isset() is to check if a variable is set with a value and that value should not be null.
• empty() is to check if a given variable is empty. The difference with isset() is, isset has null check.
• is_null() is to check whether a variable is defined as null.

I feel the effective way to communicate the difference would be with the help of a TRUTH TABLE,

	“”	“apple”	NULL	FALSE	0	undefined
empty()	TRUE	FALSE	TRUE	TRUE	TRUE	TRUE
is_null()	FALSE	FALSE	TRUE	FALSE	FALSE	ERROR
isset()	TRUE	TRUE	FALSE	TRUE	TRUE	FALSE

Ok! Now find the difference!

Code to understand the difference

difference.php

<?php
print "<br/>ISSET: <br/>";
$var = "";
print 'isset(""): ' . isset($var) . "<br/>";
$var = 'apple';
print "isset('apple'): " . isset($var) . "<br/>";
$var = null;
print "isset(null): " . isset($var) . "<br/>";
$var = FALSE;
print "isset(FALSE): " . isset($var) . "<br/>";
$var = 0;
print "isset(0): " . isset($var) . "<br/>";
print "isset(undefined): " . isset($var3) . "<br/>";

print "<br/>EMPTY: <br/>";
$var = "";
print 'empty(""): ' . empty($var) . "<br/>";
$var = 'apple';
print "empty('apple'): " . empty($var) . "<br/>";
$var = null;
print "empty(null): " . empty($var) . "<br/>";
$var = FALSE;
print "empty(FALSE): " . empty($var) . "<br/>";
$var = 0;
print "empty(0): " . empty($var) . "<br/>";
print "empty(undefined): " . empty($var1) . "<br/>";

print "<br/>IS_NULL: <br/>";
$var = "";
print 'is_null(""): ' . is_null($var) . "<br/>";
$var = 'apple';
print "is_null('apple'): " . is_null($var) . "<br/>";
$var = null;
print "is_null(null): " . is_null($var) . "<br/>";
$var = FALSE;
print "is_null(FALSE): " . is_null($var) . "<br/>";
$var = 0;
print "is_null('0'): " . is_null($var) . "<br/>";
print "is_null(undefined):" . is_null($var2) . "<br/>";

?>

Output:

ISSET:
isset(""): 1
isset('apple'): 1
isset(null):
isset(FALSE): 1
isset(0): 1
isset(undefined):

EMPTY:
empty(""): 1
empty('apple'):
empty(null): 1
empty(FALSE): 1
empty(0): 1
empty(undefined): 1

IS_NULL:
is_null(""):
is_null('apple'):
is_null(null): 1
is_null(FALSE):
is_null('0'):

Notice: Undefined variable: var2 in /Library/WebServer/Documents/test/difference.php on line 39
is_null(undefined):1

Differences

isset:

Returns true for empty string, False, 0 or an undefined variable. Returns false for null.

empty:

Returns true for null, empty string, False, 0 or an undefined variable. Returns true if there is any value.

is_null:

Returns true for null only. Returns false in all other cases. Throws warning if the variable is undefined.

Differences table

Value of $var	isset	empty	is_null
“” (empty string)	true	true	false
FALSE	true	true	false
0	true	true	false
null(declaration-only)	false	true	true
undefined variable	false	true	Warning
TRUE	true	false	false
array() (empty array)	true	true	false
‘apple’ (string value)	true	true	false
123 (numeric value)	true	true	false

References

• isset PHP manual
• empty PHP manual
• is-null PHP manual

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