array_keys
array_keys() returns the keys, numeric and string, from the array .
If a filter_value is specified, then only the keys for that value are returned. Otherwise, all the keys from the array are returned.
Parameters
An array containing keys to return.
If specified, then only keys containing this value are returned.
Determines if strict comparison (===) should be used during the search.
Return Values
Returns an array of all the keys in array .
Examples
Example #1 array_keys() example
$array = array( 0 => 100 , «color» => «red» );
print_r ( array_keys ( $array ));
$array = array( «blue» , «red» , «green» , «blue» , «blue» );
print_r ( array_keys ( $array , «blue» ));
$array = array( «color» => array( «blue» , «red» , «green» ),
«size» => array( «small» , «medium» , «large» ));
print_r ( array_keys ( $array ));
?>
The above example will output:
Array ( [0] => 0 [1] => color ) Array ( [0] => 0 [1] => 3 [2] => 4 ) Array ( [0] => color [1] => size )
See Also
- array_values() — Return all the values of an array
- array_combine() — Creates an array by using one array for keys and another for its values
- array_key_exists() — Checks if the given key or index exists in the array
- array_search() — Searches the array for a given value and returns the first corresponding key if successful
User Contributed Notes 28 notes
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.
$importantKeys = array( ‘329462291595’ => null , ‘ZZ291595’ => null );
foreach( array_keys ( $importantKeys ) as $key ) echo gettype ( $key ). «\n» ;
>
?>
will return on a 64 bits system:
integer
string
?>
but on a 32 bits system:
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:
$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.
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()
)
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.
$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
—-
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.
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:
$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
# array_keys() also return the key if it’s boolean but the boolean will return as 1 or 0. It will return empty if get NULL value as key. Consider the following array:
$a = array(
«first_index» => «This is the first element» ,
true => 3 ,
false => 2 ,
4.5 => ‘Something’ ,
«08» => 5 ,
«8» => 6 ,
NULL => ‘Null key’
);
Array
(
[ 0 ] => first_index
[ 1 ] => 1
[ 2 ] => 0
[ 3 ] => 4
[ 4 ] => 08
[ 5 ] => 8
[ 6 ] =>
)
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:
foreach( $array as $key => $val ) $flattenedKeysArray [] = $key ;
if( is_array ( $val ))
array_walk_keys ( $val , $key , $flattenedKeysArray );
>
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):
$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:
I wrote a function to get keys of arrays recursivelly.
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
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.
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):
$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’],
Php array get all index
// Before php 5.4
$array = array(1,2,3);
// since php 5.4 , short syntax
$array = [1,2,3];
// I recommend using the short syntax if you have php version >= 5.4
Used to creating arrays like this in Perl?
Looks like we need the range() function in PHP:
$array = array_merge (array( ‘All’ ), range ( ‘A’ , ‘Z’ ));
?>
You don’t need to array_merge if it’s just one range:
There is another kind of array (php>= 5.3.0) produced by
$array = new SplFixedArray(5);
Standard arrays, as documented here, are marvellously flexible and, due to the underlying hashtable, extremely fast for certain kinds of lookup operation.
Supposing a large string-keyed array
$arr=[‘string1’=>$data1, ‘string2’=>$data2 etc. ]
when getting the keyed data with
php does *not* have to search through the array comparing each key string to the given key (‘string1’) one by one, which could take a long time with a large array. Instead the hashtable means that php takes the given key string and computes from it the memory location of the keyed data, and then instantly retrieves the data. Marvellous! And so quick. And no need to know anything about hashtables as it’s all hidden away.
However, there is a lot of overhead in that. It uses lots of memory, as hashtables tend to (also nearly doubling on a 64bit server), and should be significantly slower for integer keyed arrays than old-fashioned (non-hashtable) integer-keyed arrays. For that see more on SplFixedArray :
Unlike a standard php (hashtabled) array, if you lookup by integer then the integer itself denotes the memory location of the data, no hashtable computation on the integer key needed. This is much quicker. It’s also quicker to build the array compared to the complex operations needed for hashtables. And it uses a lot less memory as there is no hashtable data structure. This is really an optimisation decision, but in some cases of large integer keyed arrays it may significantly reduce server memory and increase performance (including the avoiding of expensive memory deallocation of hashtable arrays at the exiting of the script).
When creating arrays , if we have an element with the same value as another element from the same array, we would expect PHP instead of creating new zval container to increase the refcount and point the duplicate symbol to the same zval. This is true except for value type integer.
Example:
$arr = [‘bebe’ => ‘Bob’, ‘age’ => 23, ‘too’ => 23 ];
xdebug_debug_zval( ‘arr’ );
(refcount=2, is_ref=0)
array (size=3)
‘bebe’ => (refcount=1, is_ref=0)string ‘Bob’ (length=3)
‘age’ => (refcount=0, is_ref=0)int 23
‘too’ => (refcount=0, is_ref=0)int 23
but :
$arr = [‘bebe’ => ‘Bob’, ‘age’ => 23, ‘too’ => ’23’ ];
xdebug_debug_zval( ‘arr’ );
(refcount=2, is_ref=0)
array (size=3)
‘bebe’ => (refcount=1, is_ref=0)string ‘Bob’ (length=3)
‘age’ => (refcount=0, is_ref=0)int 23
‘too’ => (refcount=1, is_ref=0)string ’23’ (length=2)
or :
$arr = [‘bebe’ => ‘Bob’, ‘age’ => [1,2], ‘too’ => [1,2] ];
xdebug_debug_zval( ‘arr’ );
(refcount=2, is_ref=0)
array (size=3)
‘bebe’ => (refcount=1, is_ref=0)string ‘Bob’ (length=3)
‘age’ => (refcount=2, is_ref=0)
array (size=2)
0 => (refcount=0, is_ref=0)int 1
1 => (refcount=0, is_ref=0)int 2
‘too’ => (refcount=2, is_ref=0)
array (size=2)
0 => (refcount=0, is_ref=0)int 1
1 => (refcount=0, is_ref=0)int 2
This function makes (assoc.) array creation much easier:
function arr (. $array )< return $array ; >
?>
It allows for short syntax like:
$arr = arr ( x : 1 , y : 2 , z : 3 );
?>
Instead of:
$arr = [ «x» => 1 , «y» => 2 , «z» => 3 ];
// or
$arr2 = array( «x» => 1 , «y» => 2 , «z» => 3 );
?>
Sadly PHP 8.2 doesn’t support this named arguments in the «array» function/language construct.