PHP Array

Remove specific element by value from an array in PHP?

Description: Write a PHP program to remove a specific element, by its value, from an array.

Rules and hints:

• Take an array with the names of some months.
• Take a variable with the value that you want to remove.
• You can use array functions or a foreach loop.

Solution 1: array_search() and unset()

array_search() finds the key of the value, and unset() removes that key. The other keys are not changed.

<?php
$months = ["January", "February", "March", "April", "May"];
$remove = "March";

$key = array_search($remove, $months);

if ($key !== false) {
    unset($months[$key]);
}

print_r($months);
?>

Output:

Solution 2: array_diff()

array_diff() removes all matching values (also if the value is many times in the array). Use array_values() to get new keys 0, 1, 2…

<?php
$months = ["January", "February", "March", "April", "May"];
$remove = "March";

$months = array_values(array_diff($months, [$remove]));

echo implode(", ", $months);
?>

Output:

Solution 3: array_filter()

<?php
$months = ["January", "February", "March", "April", "May"];
$remove = "March";

$months = array_filter($months, fn($month) => $month !== $remove);

echo implode(", ", $months);
?>

Output:

Note that solution 1 removes only the first match, but solutions 2 and 3 remove every match.

PHP Array to String Conversion (favourite colours chosen by user)

Description: Write a PHP exercise for array to string conversion.

Rules and hints:

• Create a form that accepts a name and some colors (checkboxes).
• After the submit, the data goes to another page.
• Show the user name and the selected colors as a list.
• The name and at least one color are required.

Step 1: the form (index.html)

<form action="data.php" method="post">
    Name: <input type="text" name="username" required><br><br>

    Select your favourite colors:<br>
    <input type="checkbox" name="colors[]" value="Red"> Red
    <input type="checkbox" name="colors[]" value="Green"> Green
    <input type="checkbox" name="colors[]" value="Blue"> Blue
    <input type="checkbox" name="colors[]" value="Yellow"> Yellow<br><br>

    <button type="submit">Send</button>
</form>

The name colors[] (with the square brackets) tells PHP to collect all the selected values in an array.

Step 2: data.php

<?php
$username = trim($_POST["username"] ?? "");
$colors = $_POST["colors"] ?? [];

if ($username === "" || empty($colors)) {
    exit("Name and at least one color are required.");
}

echo "Hello " . htmlspecialchars($username) . "<br>";
echo "Your favourite colors: " . htmlspecialchars(implode(", ", $colors));
?>

Output:

How it works

• $_POST["colors"] is an array of the checked boxes.
• implode(", ", $colors) converts the array to a text. See how to convert an array to a string.
• htmlspecialchars() protects the page from XSS.

How to check if an array is a subset of another in PHP?

Description: Write a PHP program to find out if an array is a subset of another array.

Rules and hints:

• The first array is large, and the second array is small.
• The second array is a subset if all of its values exist in the first array.
• You can use decision making statements.

PHP program to check if an array is a subset of another

<?php
$array1 = ["a", "1", "2", "3", "4"];
$array2 = ["a", "3"];
$array3 = ["a", "z"];

// array_diff gives the values of the first array that are missing in the second
if (count(array_diff($array2, $array1)) === 0) {
    echo "array2 is a subset of array1<br>";
} else {
    echo "array2 is not a subset of array1<br>";
}

if (count(array_diff($array3, $array1)) === 0) {
    echo "array3 is a subset of array1";
} else {
    echo "array3 is not a subset of array1";
}
?>

Output:

How it works

• array_diff($array2, $array1) returns the values of $array2 that are not in $array1.
• If nothing is missing (the count is 0), all values exist, so it is a subset.
• “z” is not in $array1, so $array3 is not a subset.

Another way: the result of array_intersect($array2, $array1) must have the same count as $array2. See PHP Array Functions.