[php] PHP: How to use array_filter() to filter array keys?

The callback function in array_filter() only passes in the array's values, not the keys.

If I have:

$my_array = array("foo" => 1, "hello" => "world");

$allowed = array("foo", "bar");

What's the best way to delete all keys in $my_array that are not in the $allowed array?

Desired output:

$my_array = array("foo" => 1);

Starting from PHP 5.6, you can use the ARRAY_FILTER_USE_KEY flag in array_filter:

$result = array_filter($my_array, function ($k) use ($allowed) {
    return in_array($k, $allowed);
}, ARRAY_FILTER_USE_KEY);

Otherwise, you can use this function (from TestDummy):

function filter_array_keys(array $array, $callback)
{
    $matchedKeys = array_filter(array_keys($array), $callback);

    return array_intersect_key($array, array_flip($matchedKeys));
}

$result = filter_array_keys($my_array, function ($k) use ($allowed) {
    return in_array($k, $allowed);
});

And here is an augmented version of mine, which accepts a callback or directly the keys:

function filter_array_keys(array $array, $keys)
{
    if (is_callable($keys)) {
        $keys = array_filter(array_keys($array), $keys);
    }

    return array_intersect_key($array, array_flip($keys));
}

// using a callback, like array_filter:
$result = filter_array_keys($my_array, function ($k) use ($allowed) {
    return in_array($k, $allowed);
});

// or, if you already have the keys:
$result = filter_array_keys($my_array, $allowed));

Last but not least, you may also use a simple foreach:

$result = [];
foreach ($my_array as $key => $value) {
    if (in_array($key, $allowed)) {
        $result[$key] = $value;
    }
}

How to get the current key of an array when using array_filter

Regardless of how I like Vincent's solution for Macek's problem, it doesn't actually use array_filter. If you came here from a search engine you maybe where looking for something like this (PHP >= 5.3):

$array = ['apple' => 'red', 'pear' => 'green'];
reset($array); // Unimportant here, but make sure your array is reset

$apples = array_filter($array, function($color) use ($&array) {
  $key = key($array);
  next($array); // advance array pointer

  return key($array) === 'apple';
}

It passes the array you're filtering as a reference to the callback. As array_filter doesn't conventionally iterate over the array by increasing it's public internal pointer you have to advance it by yourself.

What's important here is that you need to make sure your array is reset, otherwise you might start right in the middle of it.

In PHP >= 5.4 you could make the callback even shorter:

$apples = array_filter($array, function($color) use ($&array) {
  return each($array)['key'] === 'apple';
}

Naive and ugly (but seems to be faster) solution?

Only tried this in php 7.3.11 but an ugly loop seems to execute in about a third of the time. Similar results on an array with a few hundred keys. Micro-optimization, probably not useful in RW, but found it surprising and interesting:

$time = microtime(true);
$i = 100000;
while($i) {
    $my_array = ['foo' => 1, 'hello' => 'world'];
    $allowed  = ['foo', 'bar'];
    $filtered = array_filter(
        $my_array,
        function ($key) use ($allowed) {
            return in_array($key, $allowed);
        },
        ARRAY_FILTER_USE_KEY
    );
    $i--;
}
print_r($filtered);
echo microtime(true) - $time . ' on array_filter';

// 0.40600109100342 on array_filter
$time2 = microtime(true);
$i2 = 100000;
while($i2) {
    $my_array2 = ['foo' => 1, 'hello' => 'world'];
    $allowed2  = ['foo', 'bar'];
    $filtered2 = [];
    foreach ($my_array2 as $k => $v) {
        if (in_array($k, $allowed2)) $filtered2[$k] = $v;
    }
    $i2--;
}
print_r($filtered2);
echo microtime(true) - $time2 . ' on ugly loop';
// 0.15677785873413 on ugly loop

With array_intersect_key and array_flip:

var_dump(array_intersect_key($my_array, array_flip($allowed)));

array(1) {
  ["foo"]=>
  int(1)
}

If you are looking for a method to filter an array by a string occurring in keys, you can use:

$mArray=array('foo'=>'bar','foo2'=>'bar2','fooToo'=>'bar3','baz'=>'nope');
$mSearch='foo';
$allowed=array_filter(
    array_keys($mArray),
    function($key) use ($mSearch){
        return stristr($key,$mSearch);
    });
$mResult=array_intersect_key($mArray,array_flip($allowed));

The result of print_r($mResult) is

Array ( [foo] => bar [foo2] => bar2 [fooToo] => bar3 )

An adaption of this answer that supports regular expressions

function array_preg_filter_keys($arr, $regexp) {
  $keys = array_keys($arr);
  $match = array_filter($keys, function($k) use($regexp) {
    return preg_match($regexp, $k) === 1;
  });
  return array_intersect_key($arr, array_flip($match));
}

$mArray = array('foo'=>'yes', 'foo2'=>'yes', 'FooToo'=>'yes', 'baz'=>'nope');

print_r(array_preg_filter_keys($mArray, "/^foo/i"));

Output

Array
(
    [foo] => yes
    [foo2] => yes
    [FooToo] => yes
)

I needed to do same, but with a more complex array_filter on the keys.

Here's how I did it, using a similar method.

// Filter out array elements with keys shorter than 4 characters
$a = array(
  0      => "val 0", 
  "one"  => "val one", 
  "two"  => "val two", 
  "three"=> "val three", 
  "four" => "val four", 
  "five" => "val five", 
  "6"    => "val 6"
); 

$f = array_filter(array_keys($a), function ($k){ return strlen($k)>=4; }); 
$b = array_intersect_key($a, array_flip($f));
print_r($b);

This outputs the result:

Array
(
    [three] => val three
    [four] => val four
    [five] => val five
)

$elements_array = ['first', 'second'];

function to remove some array elements

function remove($arr, $data) {
    return array_filter($arr, function ($element) use ($data) {
        return $element != $data;
    });
}

call and print

print_r(remove($elements_array, 'second'));

the result Array ( [0] => first )

Here's a less flexible alternative using unset():

$array = array(
    1 => 'one',
    2 => 'two',
    3 => 'three'
);
$disallowed = array(1,3);
foreach($disallowed as $key){
    unset($array[$key]);
}

The result of print_r($array) being:

Array
(
    [2] => two
)

This is not applicable if you want to keep the filtered values for later use but tidier, if you're certain that you don't.

Perhaps an overkill if you need it just once, but you can use YaLinqo library* to filter collections (and perform any other transformations). This library allows peforming SQL-like queries on objects with fluent syntax. Its where function accepts a calback with two arguments: a value and a key. For example:

$filtered = from($array)
    ->where(function ($v, $k) use ($allowed) {
        return in_array($k, $allowed);
    })
    ->toArray();

(The where function returns an iterator, so if you only need to iterate with foreach over the resulting sequence once, ->toArray() can be removed.)

_{* developed by me}

Here is a more flexible solution using a closure:

$my_array = array("foo" => 1, "hello" => "world");
$allowed = array("foo", "bar");
$result = array_flip(array_filter(array_flip($my_array), function ($key) use ($allowed)
{
    return in_array($key, $allowed);
}));
var_dump($result);

Outputs:

array(1) {
  'foo' =>
  int(1)
}

So in the function, you can do other specific tests.

//Filter out array elements with keys shorter than 4 characters // By using Anonymous function with Closure...

function comparison($min)
{
   return function($item) use ($min) { 
      return strlen($item) >= $min;   
   }; 
}

$input = array(
  0      => "val 0",
  "one"  => "val one",
  "two"  => "val two",
  "three"=> "val three",
  "four" => "val four",  
  "five" => "val five",    
  "6"    => "val 6"    
);

$output = array_filter(array_keys($input), comparison(4));

print_r($output);

With this function you can filter a multidimensional array

function filter_array_keys($array,$filter_keys=array()){

    $l=array(&$array);
    $c=1;
    //This first loop will loop until the count var is stable//
    for($r=0;$r<$c;$r++){
        //This loop will loop thru the child element list//
        $keys = array_keys($l[$r]);

        for($z=0;$z<count($l[$r]);$z++){
            $object = &$l[$r][$keys[$z]];

            if(is_array($object)){
                $i=0;
                $keys_on_array=array_keys($object);
                $object=array_filter($object,function($el) use(&$i,$keys_on_array,$filter_keys){
                    $key = $keys_on_array[$i];
                    $i++;

                    if(in_array($key,$filter_keys) || is_int($key))return false;                
                    return true;                        
                });
            }

            if(is_array($l[$r][$keys[$z]])){
                $l[] = &$l[$r][$keys[$z]];
                $c++;
            }//IF           
        }//FOR
    }//FOR  

    return $l[0];

}

array filter function from php:

array_filter ( $array, $callback_function, $flag )

$array - It is the input array

$callback_function - The callback function to use, If the callback function returns true, the current value from array is returned into the result array.

$flag - It is optional parameter, it will determine what arguments are sent to callback function. If this parameter empty then callback function will take array values as argument. If you want to send array key as argument then use $flag as ARRAY_FILTER_USE_KEY. If you want to send both keys and values you should use $flag as ARRAY_FILTER_USE_BOTH .

For Example : Consider simple array

$array = array("a"=>1, "b"=>2, "c"=>3, "d"=>4, "e"=>5);

If you want to filter array based on the array key, We need to use ARRAY_FILTER_USE_KEY as third parameter of array function array_filter.

$get_key_res = array_filter($array,"get_key",ARRAY_FILTER_USE_KEY );

If you want to filter array based on the array key and array value, We need to use ARRAY_FILTER_USE_BOTH as third parameter of array function array_filter.

$get_both = array_filter($array,"get_both",ARRAY_FILTER_USE_BOTH );

Sample Callback functions:

 function get_key($key)
 {
    if($key == 'a')
    {
        return true;
    } else {
        return false;
    }
}
function get_both($val,$key)
{
    if($key == 'a' && $val == 1)
    {
        return true;
    }   else {
        return false;
    }
}

It will output

Output of $get_key is :Array ( [a] => 1 ) 
Output of $get_both is :Array ( [a] => 1 )