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This PHP5 tutorial discusses about PHP arrays.

What is an array?
Generally speaking an array is continuous allocation of memory of similar data types.

This means that you can define a single array variable which can hold multiple values at the same time. These multiple values are retrieved using an index number. Don’t worry if you did not get this… more explanation ahead.






The same definition mentioned above is also applicable for PHP as well, with an exception that it can store data of different data types in one array.

Memory representation of an array

In the above diagram, you can see that variable $a has been allocated 5 slots of memory in which data can be stored. Each of these slots / boxes are referred using an index number that starts from 0. A common mistake for people learning arrays is to consider the starting position of an array as 1.

Remember a rule of thumb, if an array has 5 elements then the starting index of the array is from 0 (always) and the ending index number 5 - 1, i.e. 4.

Declaring an PHP array

$a = array(); // this line creates an array by the name $a
$a = array(1,2,3); //an array declared and initialized with integer values
$a = array(’name’ => ’sunil’, ‘email’ => ‘email@domain.com’); // an associative array

PHP Array manipulation

You can store and retrieve values from an array during program execution as follows:

a. this assigns/overwrites the value stored in the first element of array $a
$a[0] = 10;

b. this assigns/overwrites the value stored in the sixth element of array $a
$a[5] = 20;

c. this assigns/overwrites the value in the last element of array $a
$a[count($a) - 1] = 100;

You will learn more about the count() function in a while.

Types of arrays in PHP

You can declare 2 types of arrays in PHP.

a. Index based arrays
An index based array will be manipulated using an index number, like $a[0], $a[2], etc.

b. Associative arrays
An associative array will be manipulated using keys, like $a[’name’], $a[’email’];

Traversing an array
Traversing an array means the logic to visit each and every element of an array using a looping structure like while() and for(,,).

a. Traversing index based arrays

<?php

$a = array(10,20,30);

for($i = 0 ; $i < count($a) ; $i++) {
echo $a[$i] . “\n”;
}

?>

Output:
1
2
3

In the above example, we use a for() looping structure to traverse all the elements of the array. count() is an array function which returns the count of number of elements in the array $a, which in our case is 3.

The reason why we subtract 1 from count(), i.e. count($a) - 1; is because the index of the last element is always N - 1. N in this case is 3.






b. Traversing an associative array

Traversing an associative array is a little different from traversing a index based array. The difference is that an associative array does not have index. It uses a key/value pair for storage.

<?php

$a = array(’name’ => ’sunil’, ‘email’ => ‘email@domain.com’); // an associative array

foreach($a as $key => $value) {
echo $key . ” : ” . $value . “\n”;
}

?>

Output:
name : sunil
email : email@domain.com

In the above example, the key and value of each element of array $a is stored in $key and $value respectively. After the first iteration, $key contains ‘name’ and $value contains ’sunil’

Multi-dimension Array

A multi-dimension array is an array within an array. This means that every element of a multi-dimension array is another array itself. Look at the memory representation below.

Declaring a multi-dimension array

Example 1:

$multi = array(
array(1,2,3),
array(99,98,97)
);

The above is an index based multi-dimension array. Look at the diagram below.

Example 2:

$multi = array(
‘first’ => array(1,2,3),
’second’ => array(99,98,97),
);

The above is an associative multi-dimension array. Look at the diagram below.

Traversing a multi-dimension array

Traversing an array means to programmatically visit each and every element of an array.

a. Traversing a index based multi-dimension array

<?php

$multi = array(
array(1,2,3),
array(99,98,97)
);

for($i = 0 ; $i < count($multi) ; $i++) {
for($j = 0 ; $j < count($multi[$i]) ; $j++) {
echo $multi[$i][$j] . ” “;
}
echo “\n”;
}

?>

Output:
1 2 3
99 98 97

b. Traversing an associative multi-dimension array

<?php

$multi = array(
‘first’ => array(1,2,3),
’second’ => array(45, 67, 98)
);

foreach($multi as $key => $value) {
echo $key . ” : “;
for($j = 0 ; $j < count($multi[$key]) ; $j++) {
echo $multi[$key][$j] . ” “;
}
echo “\n”;
}

?>

Output:
first : 1 2 3
second : 45 67 98

Please feel free to leave behind any comments or questions that you might have.

Related Posts

• Tutorial 1 - Introduction to PHP Web Programming
• Tutorial 2 - PHP Variables
• Tutorial 3 - PHP Strings
• PHP5 Tutorial - Magic Methods - __toString() method
• PHP5 OOPS Tutorial __get() and __set()

Filed under: PHP, PHP Tutorials