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array_filter

(PHP 4 >= 4.0.6, PHP 5, PHP 7, PHP 8)

array_filterFilters elements of an array using a callback function

Description

array_filter(array $array, ?callable $callback = null, int $mode = 0): array

Iterates over each value in the array passing them to the callback function. If the callback function returns true, the current value from array is returned into the result array.

Array keys are preserved, and may result in gaps if the array was indexed. The result array can be reindexed using the array_values() function.

Parameters

array

The array to iterate over

callback

The callback function to use

If no callback is supplied, all empty entries of array will be removed. See empty() for how PHP defines empty in this case.

mode

Flag determining what arguments are sent to callback:

  • ARRAY_FILTER_USE_KEY - pass key as the only argument to callback instead of the value
  • ARRAY_FILTER_USE_BOTH - pass both value and key as arguments to callback instead of the value
Default is 0 which will pass value as the only argument to callback instead.

Return Values

Returns the filtered array.

Changelog

Version Description
8.0.0 callback is nullable now.

Examples

Example #1 array_filter() example

<?php
function odd($var)
{
    
// returns whether the input integer is odd
    
return $var 1;
}

function 
even($var)
{
    
// returns whether the input integer is even
    
return !($var 1);
}

$array1 = ['a' => 1'b' => 2'c' => 3'd' => 4'e' => 5];
$array2 = [6789101112];

echo 
"Odd :\n";
print_r(array_filter($array1"odd"));
echo 
"Even:\n";
print_r(array_filter($array2"even"));
?>

The above example will output:

Odd :
Array
(
    [a] => 1
    [c] => 3
    [e] => 5
)
Even:
Array
(
    [0] => 6
    [2] => 8
    [4] => 10
    [6] => 12
)

Example #2 array_filter() without callback

<?php

$entry 
= [
    
=> 'foo',
    
=> false,
    
=> -1,
    
=> null,
    
=> '',
    
=> '0',
    
=> 0,
];

print_r(array_filter($entry));
?>

The above example will output:

Array
(
    [0] => foo
    [2] => -1
)

Example #3 array_filter() with mode

<?php

$arr 
= ['a' => 1'b' => 2'c' => 3'd' => 4];

var_dump(array_filter($arr, function($k) {
    return 
$k == 'b';
}, 
ARRAY_FILTER_USE_KEY));

var_dump(array_filter($arr, function($v$k) {
    return 
$k == 'b' || $v == 4;
}, 
ARRAY_FILTER_USE_BOTH));
?>

The above example will output:

array(1) {
  ["b"]=>
  int(2)
}
array(2) {
  ["b"]=>
  int(2)
  ["d"]=>
  int(4)
}

Notes

Caution

If the array is changed from the callback function (e.g. element added, deleted or unset) the behavior of this function is undefined.

See Also

  • array_intersect() - Computes the intersection of arrays
  • array_map() - Applies the callback to the elements of the given arrays
  • array_reduce() - Iteratively reduce the array to a single value using a callback function
  • array_walk() - Apply a user supplied function to every member of an array

add a note add a note

User Contributed Notes 5 notes

up
530
Anonymous
8 years ago
If you want a quick way to remove NULL, FALSE and Empty Strings (""), but leave values of 0 (zero), you can use the standard php function strlen as the callback function:
eg:
<?php

// removes all NULL, FALSE and Empty Strings but leaves 0 (zero) values
$result = array_filter( $array, 'strlen' );

?>
up
17
nicolaj dot knudsen at gmail dot com
4 years ago
If you like me have some trouble understanding example #1 due to the bitwise operator (&) used, here is an explanation.

The part in question is this callback function:

<?php
function odd($var)
{
   
// returns whether the input integer is odd
   
return($var & 1);
}
?>

If given an integer this function returns the integer 1 if $var is odd and the integer 0 if $var is even.
The single ampersand, &, is the bitwise AND operator. The way it works is that it takes the binary representation of the two arguments and compare them bit for bit using AND. If $var = 45, then since 45 in binary is 101101 the operation looks like this:

45 in binary: 101101
1 in binary:  000001
              ------
result:       000001

Only if the last bit in the binary representation of $var is changed to zero (meaning that the value is even) will the result change to 000000, which is the representation of zero.
up
29
marc dot vanwoerkom at fernuni-hagen dot de
17 years ago
Some of PHP's array functions play a prominent role in so called functional programming languages, where they show up under a slightly different name:

<?php
  array_filter
() -> filter(),
 
array_map() -> map(),
 
array_reduce() -> foldl() ("fold left")
?>

Functional programming is a paradigm which centers around the side-effect free evaluation of functions. A program execution is a call of a function, which in turn might be defined by many other functions. One idea is to use functions to create special purpose functions from other functions.

The array functions mentioned above allow you compose new functions on arrays.

E.g. array_sum = array_map("sum", $arr).

This leads to a style of programming that looks much like algebra, e.g. the Bird/Meertens formalism.

E.g. a mathematician might state

  map(f o g) = map(f) o map(g)

the so called "loop fusion" law.

Many functions on arrays can be created by the use of the foldr() function (which works like foldl, but eating up array elements from the right).

I can't get into detail here, I just wanted to provide a hint about where this stuff also shows up and the theory behind it.
up
9
marc dot gray at gmail dot com
7 years ago
My favourite use of this function is converting a string to an array, trimming each line and removing empty lines:

<?php
$array
= array_filter(array_map('trim', explode("\n", $string)), 'strlen');
?>

Although it states clearly that array keys are preserved, it's important to note this includes numerically indexed arrays. You can't use a for loop on $array above without processing it through array_values() first.
up
0
ASchmidt at Anamera dot net
19 days ago
Depending on the intended meanings of your "empty" array values, e.g., null and empty string, vs. an integer 0 or a boolean false, be mindful of the result of different filters.

<?php
declare(strict_types=1);

$array = array( 'null' => null, 'nullstring' => '''intzero' => 0'stringzero' => '0', 'false' => false, 'stringfalse' => 'false', );

// Removes null, null-string -- but also FALSE!
$filtered1 = array_filter( $array, 'strlen' );

// Removes only null.
$filtered2 = array_filter( $array, function( $v ) { return !is_null( $v ); } );

// Removes null and null-string. Keeps FALSE and 0.
$filtered3 = array_filter( $array, function( $v ) { return !( is_null( $v) or '' === $v ); } );

var_dump( $array, $filtered1, $filtered2, $filtered3 );
?>

Results in:

array (size=3)
  'intzero' => int 0
  'stringzero' => string '0' (length=1)
  'stringfalse' => string 'false' (length=5)

array (size=5)
  'nullstring' => string '' (length=0)
  'intzero' => int 0
  'stringzero' => string '0' (length=1)
  'false' => boolean false
  'stringfalse' => string 'false' (length=5)

array (size=4)
  'intzero' => int 0
  'stringzero' => string '0' (length=1)
  'false' => boolean false
  'stringfalse' => string 'false' (length=5)
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