Arrays are very common in PHP and if you do any type of coding in WordPress you’re likely to encounter them. In this article I explain the basics of what an array is and how to work with them.

Arrays

An array is actually an ordered map. A map is a type that associates values to keys.

PHP Manual

An array can be created using the array() language construct. It takes any number of comma-separated key => value pairs as arguments.

array(
    key  => value,
    key2 => value2,
    key3 => value3,
    ...
)

Array Functions

You can find all array() functions in the PHP Manual, https://www.php.net/manual/en/ref.array.php. The functions you’ll likely use most often are:

array_diff() :: Compares an array against other arrays and returns the values that are not present in any of the others.

array_intersect() :: Compares an array against other arrays and returns the values that are present in all of them.

array_key_exists() :: Checks if a given key or index exists

array_merge() :: Merges the elements of one or more arrays

array_push() :: Push elements onto the end of an array

array_replace_recursive() :: Replaces elements from passed arrays into the first array recursively

array_reverse() :: Return an array with elements in reverse order

array_slice() :: Extract a slice of an array

array_unique() :: Removes duplicates from an array

array_unshift() :: Prepend elements to the beginning of an array

count() :: Counts all elements in an array

in_array() :: Checks if a value exists in an array

Array Functions with callbacks

array_filter() :: Filters elements of an array using a callback function. If no callback is supplied, all empty entries of arrays will be removed.

array_map() :: Applies the callback to the elements of the given arrays

array_udiff() :: Computes the difference of arrays by using a callback function for data comparison

Coding Examples

Recently Viewed Widget

I implemented my own version of “recently viewed” using the existing Woo widget data. In this code I use:

  • array_map() to cast all values as integers
  • array_filter() to remove empty elements
  • array_reverse() to show the element added last at beginning
$viewed_products = 
! empty( $_COOKIE['woocommerce_recently_viewed'] ) 
? (array) explode( '|', wp_unslash( $_COOKIE['woocommerce_recently_viewed'] ) ) 
: array(); 
		
$viewed_products = 
array_reverse( 
array_filter( 
array_map( 'absint', $viewed_products ) ) );

Merging arrays in a filter

This is a very common example. For WP filters containing arrays as return, you’d use array_merge to merge the original array and your own new array. In this example I needed to register new search query vars in a class object:

public function add_query_vars( $vars ) {
     return array_merge(
             $this->new_vars, 
             $vars 
     ); 		
}

add_filter( 'query_vars', array( $this, 'add_query_vars'  )  );

Insert a value after a specific key

This is a useful function I found on github to insert a value after a specific key in an array. It uses:

  • array_slice to slice the existing array at the point of insertion
  • arrray_merge to recombine the elements
/**
 * Insert a value or key/value pair after a specific key in an array.  If key doesn't exist, value is appended
 * to the end of the array.
 *
 * @param array $array
 * @param string $key
 * @param array $new
 *
 * @return array
 * Source https://gist.github.com/wpscholar/0deadce1bbfa4adb4e4c
 */
function array_insert_after( array $array, $key, array $new, $strict = true ) {
	$keys = array_keys( $array );
	$index = array_search( $key, $keys, $strict);
	$pos = false === $index ? count( $array ) : $index + 1;

	return array_merge( array_slice( $array, 0, $pos ), $new, array_slice( $array, $pos ) );
}

WordPress Functions

WordPress Array Functions

WP also has its own functions to handle arrays. Below is a non-exhaustive list from the WP Code Reference:

_wp_array_get() :: Accesses an array in depth based on path of keys

_wp_array_set() :: Sets an array in depth based on a path of keys

wp_kses_array_lc() :: Converts the keys of an array to lowercase

wp_is_numeric_array() :: Determines if it is a numeric-indexed array

wp_array_slice_assoc() :: Extract a slice, given a list of keys

__return_empty_array() :: returns an empty array. Useful for filters.

WordPress Array + Object Functions

WP also offers functions which work for both arrays and objects. Below is a non-exhaustive list of functions I use regularly:

wp_list_pluck() :: Plucks a certain field out of each object or array in an array.

Coding Examples

Flatten an ACF repeater field array

This is a quick way to extract an array from an ACF repeater field, for example, to use in a select dropdown:

$multi_array = get_field( 'repeater_field' );
$array = wp_list_pluck( $multi_array, 'sub_field' );

Changes:
Array (
    [0] => Array ( [sub_field] => lorem ipsum )
    [1] => Array ( [sub_field] => dolor sit )
)
Into:
Array (
    [0] => lorem ipsum
    [1] => dolor sit
)

Interested in PHP? Got a great array function or a query? Let me know in the comments below!

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