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 $sub){
            if(is_array($sub)){
                $output = ($search_value !== null ? array_merge($output, recursive_keys($sub, $search_value)) : array_merge($output, recursive_keys($sub))) ;
            }
        }
        return $output ;
    }
?>

I hope it will be usefull

Regards

[Editor's note: For a complete solution to the printing of complex structures or hashes, see the PEAR::Var_Dump package: http://pear.php.net/package-info.php?pacid=103 , use "pear install Var_Dump" to get it]

This function will print all the keys of a multidimensional array in html tables.
It will help to debug when you don?t have control of depths.

<?php
function show_keys($ar){

   echo "<table width='100%' border='1' bordercolor='#6699CC' cellspacing='0' cellpadding='5'><tr valign='top'>";

      foreach ($ar as $k => $v ) {

         echo "<td align='center' bgcolor='#EEEEEE'>
           <table border='2' cellpadding='3'><tr><td bgcolor='#FFFFFF'><font face='verdana' size='1'>
              " . $k . "
           </font></td></tr></table>";

           if (is_array($ar[$k])) {
              show_keys ($ar[$k]);
         }

         echo "</td>";

      }

   echo "</tr></table>";

}

// Multidimensional array ->
$arvore = array();
$arvore['1'] = array();
$arvore['1']['1.1'] = array('1.1.1', '1.1.2', '1.1.3');
$arvore['1']['1.2'] = array('1.2.1', '1.2.2', '1.2.3');
$arvore['1']['1.3'] = array('1.3.1', '1.3.2', '1.3.3');
$arvore['2'] = array();
$arvore['2']['2.1'] = array('2.1.1', '2.1.2', '2.1.3');
$arvore['2']['2.2'] = array('2.2.1', '2.2.2', '2.2.3');
$arvore['2']['2.3'] = array('2.3.1', '2.3.2', '2.3.3');
$arvore['3'] = array();
$arvore['3']['3.1'] = array('3.1.1', '3.1.2', '3.1.3');
$arvore['3']['3.2'] = array('3.2.1', '3.2.2', '3.2.3');
$arvore['3']['3.3'] = array('3.3.1', '3.3.2'=>array('3.3.2.1', '3.3.2.2'), '3.3.3');
// <-

show_keys($arvore);
?>

Note, that using array_key_exists() is rather inefficient. The overhead associated with calling a function makes it slower, than using isset($array[$key]), instead of array_key_exists($key, $array)
using isset() is usually about 1.3 times faster, according to my tests.

I was looking for a function that simply unset a variable amout of values from a one-dimensional array by key. I ended up with this (returns the array itself if no further parameter than the array is given, false with no params - does not change the source array) 

usage: array_remove(array $input [, mixed key ...])

<?php

  function array_remove() {
    if ($stack = func_get_args()) {
      $input = array_shift($stack);
      foreach ($stack as $key) {
        unset($input[$key]);
      }
      return $input;
    }
    return false;
  }

?>

Test:

<?php
  $a = array('a'=>'fun', 'b'=>3.14, 'sub'=> array('1', '2', '3'), 'd'=>'what', 'e' => 'xample', 5 => 'x');
  print_r($a);
  print_r(array_remove($a, 'd', 'b', 5, 'sub'));
?>

Output: 

Array
(
    [a] => fun
    [b] => 3.14
    [sub] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

    [d] => what
    [e] => xample
    [5] => x
)
Array
(
    [a] => fun
    [e] => xample
)

Hope this helps someone.

Here's a function I needed to collapse an array, in my case from a database query.  It takes an array that contains key-value pairs and returns an array where they are actually the key and value.

<?php

function array_collapse($arr, $x, $y) {
    $carr = array();
    while ($el = current($arr)) {
        $carr[ $el[$x] ] = $el[$y];
        next($arr);
    }
    return $carr;
}

?>

Example usage (pseudo-database code):

<?php

$query = db_query('SELECT name, value FROM properties');

$result = db_returnAll($query);

/* This will return an array like so:

[
   ['name' -> 'color', 'value' -> 'blue'],
   ['name' -> 'style', 'value' -> 'wide-format'],
   ['name' -> 'weight', 'value' -> 3.6],
   ['name' -> 'name', 'value' -> 'Waerdthing']
]

*/

$propArr = array_collapse($result, 'name', 'value');

/* Now this array looks like:

[
   ['color' -> 'blue'],
   ['style' -> 'wide-format'],
   ['weight' -> 3.6],
   ['name' -> 'Waerdthing'],

*/

?>

I found this handy for using with json_encode and am using it for my project http://squidby.com

A nice little trick to get all of the keys who have some type of value:

$keys = array_keys($array, !null);