array_keys

It's worth noting that if you have keys that are long integer, such as '329462291595', they will be considered as such on a 64bits system, but will be of type string on a 32 bits system.

for example:
<?php 

$importantKeys = array('329462291595' =>null, 'ZZ291595' => null);

foreach(array_keys($importantKeys) as $key){
    echo gettype($key)."\n";
}

?>

will return on a 64 bits system:
<?php 
    integer
    string
?>

but on a 32 bits system:
<?php 
    string
    string
?>

I hope it will save someone the huge headache I had :)

Here's how to get the first key, the last key, the first value or the last value of a (hash) array without explicitly copying nor altering the original array:

<?php
  $array = array('first'=>'111', 'second'=>'222', 'third'=>'333');

  // get the first key: returns 'first'
  print array_shift(array_keys($array));

  // get the last key: returns 'third'
  print array_pop(array_keys($array));

  // get the first value: returns '111'
  print array_shift(array_values($array));

  // get the last value: returns '333'
  print array_pop(array_values($array));
?>

There's a lot of multidimensional array_keys function out there, but each of them only merges all the keys in one flat array.

Here's a way to find all the keys from a multidimensional  array while keeping the array structure. An optional MAXIMUM DEPTH parameter can be set for testing purpose in case of very large arrays.

NOTE: If the sub element isn't an array, it will be ignore.

<?php
function array_keys_recursive($myArray, $MAXDEPTH = INF, $depth = 0, $arrayKeys = array()){
       if($depth < $MAXDEPTH){
            $depth++;
            $keys = array_keys($myArray);
            foreach($keys as $key){
                if(is_array($myArray[$key])){
                    $arrayKeys[$key] = array_keys_recursive($myArray[$key], $MAXDEPTH, $depth);
                }
            }
        }

        return $arrayKeys;
    }
?>

EXAMPLE:
input:
array(
    'Player' => array(
        'id' => '4',
        'state' => 'active',
    ),
    'LevelSimulation' => array(
        'id' => '1',
        'simulation_id' => '1',
        'level_id' => '1',
        'Level' => array(
            'id' => '1',
            'city_id' => '8',
            'City' => array(
                'id' => '8',
                'class' => 'home',
            )
        )
    ),
    'User' => array(
        'id' => '48',
        'gender' => 'M',
        'group' => 'user',
        'username' => 'Hello'
    )
)

output:
array(
    'Player' => array(),
    'LevelSimulation' => array(
        'Level' => array(
            'City' => array()
        )
    ),
    'User' => array()
)

If an array is empty (but defined), or the $search_value is not found in the array, an empty array is returned (not false, null, or -1). This may seem intuitive, especially given the documentation says an array is returned, but I needed to sanity test to be sure:

<?php

$emptyArray = array();
var_dump(array_keys($emptyArray,99)); // array (size=0) \ empty

$filledArray = array(11,22,33,42);
var_dump(array_keys($filledArray,99)); // array (size=0) \ empty

?>

Keys from multi dimensional array to simple array

Want to traverse an multi dimensional array and get the keys back in a single dimensional array? This will do the trick:

<?php

    public function array_walk_keys($array, $parentKey = null, &$flattened_array = null)
    {
        if(!is_array($array))
            return $array;
        
        foreach( $array as $key => $val ) {
            $flattenedKeysArray[] = $key;
            
            if(is_array($val))
                array_walk_keys($val, $key, $flattenedKeysArray);
        }

        return $flattenedKeysArray;
    }

It is worth noting that array_keys does not maintain the data-type of the keys when mapping them to a new array.  This created an issue with in_array and doing  a lookup on characters from a string.  NOTE:  my lookup $array has a full map of numbers and characters - upper and lower - to do an simple faux encryption with.

<?php
$array = array(
     'e' => 'ieio'
    ,'1' => 'one'
    ,'2' => 'two'
    ,'0' => 'zero'
);
var_dump($array);
$keys = array_keys($array);
var_dump($keys);

$string = '1e0';
for ($i = 0; $i < strlen($string); $i++) {
    if (in_array($string[$i],$keys,'strict')) echo 'dude ';
    else echo 'sweet ';
}
?>

Outputs:
array (size=4)
  'e' => string 'ieio' (length=4)
  1 => string 'one' (length=3)
  2 => string 'two' (length=3)
  0 => string 'zero' (length=4)

array (size=4)
  0 => string 'e' (length=1)
  1 => int 1
  2 => int 2
  3 => int 0

sweet dude sweet 

----  
expected to see:
dude dude dude

Since 5.4 STRICT standards dictate that you cannot wrap array_keys in a function like array_shift that attempts to reference the array.  

Invalid:
echo array_shift( array_keys( array('a' => 'apple') ) );

Valid:
$keys = array_keys( array('a' => 'apple') );
echo array_shift( $keys );

But Wait! Since PHP (currently) allows you to break a reference by wrapping a variable in parentheses, you can currently use:

echo array_shift( ( array_keys( array('a' => 'apple') ) ) );

However I would expect in time the PHP team will modify the rules of parentheses.

might be worth noting in the docs that not all associative (string) keys are a like, output of the follow bit of code demonstrates - might be a handy introduction to automatic typecasting in php for some people (and save a few headaches):

<?php
$r = array("0"=>"0","1"=>"1","" =>"2"," "=>"3");
echo 'how php sees this array: array("0"=>"0","1"=>"1","" =>"2"," "=>"3")',"\n-----------\n";
var_dump($r); print_r($r); var_export($r);
echo "\n-----------\n",'var_dump("0","1",""," ") = ',"\n-----------\n";
var_dump("0","1",""," ");
?>

OUTPUTS:

how php sees this array: array("0"=>"0","1"=>"1","" =>"2"," "=>"3")
-----------
array(4) {
  [0]=>
  string(1) "0"
  [1]=>
  string(1) "1"
  [""]=>
  string(1) "2"
  [" "]=>
  string(1) "3"
}
Array
(
    [0] => 0
    [1] => 1
    [] => 2
    [ ] => 3
)
array (
  0 => '0',
  1 => '1',
  '' => '2',
  ' ' => '3',
)
-----------
var_dump("0","1",""," ") =
-----------
string(1) "0"
string(1) "1"
string(0) ""
string(1) " "

Post By  Sven (59892) has to be changed

$array = array('first'=>'111', 'second'=>'222', 'third'=>'333');

$rarray = array_keys($array);

print array_shift($rarray); // first

print array_pop($rarray); //thrid

print array_shift($rarray); //second

print array_pop($rarray); // no result

Code below is not valid from 5.4.0 

print array_shift(array_keys($array)); Throws Strict Standards Error

Sven code works fine till 5.3.29. From 5.4.0  the standards have been changed and results too differ.

Note :
Strict Standards :  Only variables should be passed by reference

Please have look of the code in different versions

http://sandbox.onlinephpfunctions.com/code/24b5fddf14b635f1e37db69a7edffc2cbbed55e1

http://sandbox.onlinephpfunctions.com/code/f695e8f81e906b4f062b66cf9b3b83b6b620464c

Sorry for my english...

I wrote a function to get keys of arrays recursivelly...

<?php
    function recursive_keys($input, $search_value = null){

        $output = ($search_value !== null ? array_keys($input, $search_value) : array_keys($input)) ;
        foreach($input as $sub