The while loop is the simplest of the loops. It executes a block of code until the expression to evaluate returns false. Let's see one example:

<?php
$i = 1;
while ($i < 4) {
    echo $i . " ";
    $i++;
}

In the preceding example, we define a variable with value 1. Then we have a while clause in which the expression to evaluate is $i < 4. This loop executes the content of the block of code until that expression is false. As you can see, inside the loop we are incrementing the value of $i by 1 each time, so the loop ends after 4 iterations. Check the output of that script and you will see "0 1 2 3". The last value printed is 3, so at that time the value of $i was 3. After that, we increased its value to 4, so when the while clause evaluates if $i < 4, the result is false.

The do…while loop is very similar to while in the sense that it evaluates an expression each time, and will execute the block of code until that expression is false. The only difference is that when this expression is evaluated, the while clause evaluates the expression before executing the code, so sometimes, we might not even enter the loop if the expression evaluates to false the very first time. On the other hand, do…while evaluates the expression after it executes its block of code, so even if the expression is false from the very beginning, the loop will be executed at least once.

<?php
echo "with while: ";
$i = 1;
while ($i < 0) {
    echo $i . " ";
    $i++;
}
echo "with do-while: ";
$i = 1;
do {
    echo $i . " ";
    $i++;
} while ($i < 0);

The preceding piece of code defines two loops with the same expression and block of code, but if you execute them, you will see that only the code inside the do…while is executed. In both cases, the expression is false since the beginning, so while does not even enter the loop, whereas the do…while enters the loop once.

The for loop is the most complex of the four loops. It defines an initialization expression, an exit condition, and the end of an iteration expression. When PHP first encounters the loop, it executes what is defined as the initialization expression. Then, it evaluates the exit condition and if it resolves to true, it enters the loop. After executing everything inside the loop, it executes the end of the iteration expression. Once done, it evaluates the end condition again, going through the loop code and the end of the iteration expression, until it evaluates to false. As always, an example will clarify it:

<?php
for ($i = 1; $i < 10; $i++) {
    echo $i . " ";
}

The initialization expression is $i = 1, and is executed only the first time. The exit condition is $i < 10, and it is evaluated at the beginning of each iteration. The end of the iteration expression is $i++, which is executed at the end of each iteration. This example prints the numbers from 1 to 9. Another more common usage of the for loop is with arrays:

<?php
$names = ['Harry', 'Ron', 'Hermione'];
for ($i = 0; $i < count($names); $i++) {
    echo $names[$i] . " ";
}

In this example, we have an array of names. Since it is defined as a list, its keys will be 0, 1, and 2. The loop initializes the variable $i to 0, and it iterates until the value of $i is not less than the number of elements in the array, that is, 3. In the first iteration, $i is 0, in the second, it is 1, and in the third one it is equal to 2. When $i is 3, it will not enter the loop, as the exit condition evaluates to false.

On each iteration, we print the content of the position $i of the array, hence the result of this code will be all three names in the array.

The last, but not least, type of loop is foreach. This loop is exclusive for arrays, and it allows you to iterate an array entirely, even if you do not know its keys. There are two options for the syntax, as you can see in the following examples:

<?php
$names = ['Harry', 'Ron', 'Hermione'];
foreach ($names as $name) {
    echo $name . " ";
}
foreach ($names as $key => $name) {
    echo $key . " -> " . $name . " ";
}

The foreach loop accepts an array—in this case $names—and it specifies a variable which will contain the value of the entry of the array. You can see that we do not need to specify any end condition, as PHP will know when the array has been iterated. Optionally, you can specify a variable that contains the key of each iteration, as in the second loop.

The foreach loops are also useful with maps, where the keys are not necessarily numeric. The order in which PHP iterates the array will be the same order that you used to insert the contents in the array.

Let's use some loops in our application. We want to show the available books in our home page. We have the list of books in an array, so we will have to iterate all of them with a foreach loop, printing some information from each one. Append the following code to the body tag in index.php:

<?php endif;
    $books = [
        [
            'title' => 'To Kill A Mockingbird',
            'author' => 'Harper Lee',
            'available' => true,
            'pages' => 336,
            'isbn' => 9780061120084
        ],
        [
            'title' => '1984',
            'author' => 'George Orwell',
            'available' => true,
            'pages' => 267,
            'isbn' => 9780547249643
        ],
        [
            'title' => 'One Hundred Years Of Solitude',
            'author' => 'Gabriel Garcia Marquez',
            'available' => false,
            'pages' => 457,
            'isbn' => 9785267006323
        ],
    ];
?>
<ul>
<?php foreach ($books as $book): ?>
        <li>
            <i><?php echo $book['title']; ?></i>
            - <?php echo $book['author']; ?>
<?php if (!$book['available']): ?>
          <b>Not available</b>
<?php endif; ?>
        </li>
<?php endforeach; ?>
    </ul>

The highlighted code shows a foreach loop using the : notation as well, which is better when mixing it with HTML. It iterates all of the $books array, and for each book, it prints some information as an HTML list. Notice also that we have a conditional inside a loop, which is perfectly fine. Of course, this conditional will be executed for each entry in the array, so you should keep the block of code of your loops as simple as possible.