In PHP, an ordered array is a data structure representing a list of ordered, stored data. The location of an element in the array is known as its index. The elements in an ordered array are arranged in ascending numerical order starting with zero.

In PHP, an ordered array can be constructed with the built-in PHP function: array(). The array() function returns an array. Each of the arguments with which the function was invoked becomes an element in the array (in the order they were passed in).

Pass in arguments by including them, comma-separated, between the parentheses.

In PHP, nested arrays are arrays that contain other arrays as elements. Chained operations can be used to access and/or change elements within a nested array.

The built-in print_r() function outputs arrays in a human readable format.

To use the function, place an array or a variable with an array as its value in between the parentheses.

The PHP built-in array_push() function takes an array as its first argument. The arguments that follow are elements to be added to the array.

The function adds each of the elements to the end of the array and returns the new number of elements in the array.

To use the function place an array or a variable with an array as its value in between the parentheses.

The built-in PHP count() function takes an array as its argument and returns the number of elements in that array.

To use the function place an array or a variable with an array as its value in between the parentheses.

In PHP, arrays can also be constructed using short array syntax. To use this syntax, simply wrap comma-separated elements in a set of square brackets.

The PHP built-in array_pop() function takes an array as its argument. It permanently removes the last element of an array and returns the removed element.

To use the function place an array or a variable with an array as its value in between the parentheses.

The PHP built-in array_unshift() function takes an array as its first argument. The arguments that follow are elements to be added to the array.

The function adds each of the elements to the beginning of the array and returns the new number of elements in the array.

To use the function, place an array or a variable with an array as its value in between the parentheses.

The built-in implode() function makes a string from an array. The function takes two arguments: a string to place between each element of an array and the array to be joined together.

The function returns a single string with the string argument, known as the $glue, inserted between each element in the array, known as the $pieces.

In PHP, the individual elements in an array can be accessed using the array variable’s name, and the location index surrounded by square brackets [].

Remember that in PHP the location index starts at zero.

The PHP built-in array_shift() function takes an array as its argument, permanently removes the first element from the array, and returns the removed element. All the elements in the array will be shifted to an index one smaller than their previous index.

To use the function, place an array or a variable with an array as its value in between the parentheses.

In PHP, individual elements in an array can easily be reassigned. To do this, we access the location of an element in the array using square brackets ([]), and then assign a new value using the assignment operator (=).

In PHP, we can access the value that a given key points to using square brackets ([]) and the key. To add new elements to an associative array, we can use the assignment operator (=). We can also change existing elements using the same syntax that adds new array elements.

PHP arrays can be assigned or passed by value or by reference. PHP arrays that are passed by reference use the reference sigil (&). Here are some of the the important differences between the two:

• Passed by value: this creates two variables which hold copies of the same value but remain independent entities.

• Passed by reference: this creates two variable names (aliases) which point to the same space in memory. They cannot be modified separately!

In PHP, associative arrays are map-like structures, where keys are associated with values. When we need to access a specific value, we can use the associated key to find it.

In a PHP ordered array, the index locations are the keys. However, the PHP array type also allows us to assign meaningful keys to values. These data structures are called associative arrays.

For example, in the following associative array table, the key "num_legs" points to the value 4, and the key "material" points to the value "wood":

$table = ["num_legs" => 4, "material" => "wood"];

In PHP, the union (+) operator takes two array operands and returns a new array with any unique keys from the second array appended to the first array.

A key=>value pair in a PHP associative array can be removed entirely using the PHP unset() function.

If the key used does not exist in the array, then nothing happens.

Associative arrays can use integers or strings as keys.

In PHP, ordered arrays are just arrays in which integer keys have been assigned to the values automatically. For example, the first element is associated with key 0, the second with 1, etc. Even though ordered arrays are the same structure as associative arrays, it’s recommended that you treat the two separately.

When adding an element to an array without specifying a key, PHP associates it with the “next” key. If no integer keys have been used, it will associate it with the key 0, otherwise it will associate it one more than the largest integer used so far.

In PHP, associative arrays are collections of key => value pairs, where the key must be either a string or an integer and the value can be of any type.

The => operator is used to associate a key with its value. For example:

$grades = ["John" => 91, "Sally" => 94];

The associative array grades is defined using short array syntax. The key "John" points to the value 91, and the key "Sally" points to the value 94.

We can also define this associative array using array(). For example:

$grades = array(
    "John" => 91,
    "Sally" => 94
);