Array Functions
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PHP key() Function

Topic: PHP Array ReferencePrev|Next

Description

The key() function fetch the key or index of the current element in an array.

The following table summarizes the technical details of this function.

Return Value: Returns the key of the current element in an array. Returns NULL if the array is empty or array's internal pointer points beyond the end of the elements list.
Changelog: Since PHP 7.0.0, array is always passed by value to this function. Prior to this version, it was passed by reference if possible, and by value otherwise.
Version: PHP 4+

Tip: Every array has an internal pointer that points to the current element in the array. When a new array is created, the current pointer is initialized to reference the first element in the array.


Syntax

The basic syntax of the key() function is given with:

key(array);

The following example shows the key() function in action.

<?php
// Sample array
$alphabets = array("a"=>"apple", "b"=>"ball", "c"=>"cat", "d"=>"dog");

// Getting the current element's key
echo key($alphabets); // Prints: a

/* Moving the array's internal pointer to the next 
element then fetch the key */
next($alphabets);
echo key($alphabets); // Prints: b
?>

Parameters

The key() function accepts the following parameters.

Parameter Description
array Required. Specifies the array to work on.

More Examples

Here're some more examples showing how key() function actually works:

You can also use this function with indexed array to fetch index of current element.

<?php
// Sample array
$colors = array("red", "green", "blue", "orange", "yellow", "black");

// Getting the current element's key
echo key($colors); // Prints: 0

/* Moving the array's internal pointer to the next 
element then fetch the key */
next($colors);
echo key($colors); // Prints: 1
?>

The key() function is commonly used along with the following functions:

  • current() – Returns the value of the current element in an array.
  • next() – Moves the internal pointer of an array to the next element, and returns its value.
  • prev() – Moves the internal pointer of an array to the previous element, and returns its value.
  • end() – Moves the internal pointer of an array to its last element, and returns its value.
  • reset() – Set the internal pointer of an array to its first element, and returns its value.

Here's an example that demonstrates how these functions basically work:

<?php
// Sample array
$colors = array("red", "green", "blue", "orange", "yellow", "black");

// Getting the values 
echo current($colors);  // Prints: red
echo end($colors);      // Prints: black
echo current($colors);  // Prints: black
echo prev($colors);     // Prints: yellow
echo current($colors);  // Prints: yellow
echo next($colors);     // Prints: black
echo current($colors);  // Prints: black
echo reset($colors);    // Prints: red
echo current($colors);  // Prints: red

// Getting the current element's key
echo key($colors);      // Prints: 0
?>
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