4.12.1. Problem

4.12.2. Solution

if (in_array($array, $value)) {
    // an element has $value as its value in array $array
}

4.12.3. Discussion

Use in_array( ) to check if an element of an array holds a value:

$book_collection = array('Emma', 'Pride and Prejudice', 'Northhanger Abbey');
$book = 'Sense and Sensibility';

if (in_array($book_collection, $book)) { 
    echo 'Own it.';
} else {
    echo 'Need it.';
}

$array = array(1, '2', 'three');

in_array(0, $array);        // true!
in_array(0, $array, true);  // false
in_array(1, $array);        // true
in_array(1, $array, true);  // true
in_array(2, $array);        // true
in_array(2, $array, true);  // false

The first check, in_array(0, $array), evaluates to true because to compare the number 0 against the string three, PHP casts three to an integer. Since three isn't a numeric string, as is 2, it becomes 0. Therefore, in_array( ) thinks there's a match.

Consequently, when comparing numbers against data that may contain strings, it's safest to use a strict comparison.

$book_collection = array('Emma',
                         'Pride and Prejudice',
                         'Northhanger Abbey');

// convert from numeric array to associative array
$book_collection = array_flip($book_collection);
$book = 'Sense and Sensibility';

if (isset($book_collection[$book])) { 
    echo 'Own it.';
} else {
    echo 'Need it.';
}

Note that doing this condenses multiple keys with the same value into one element in the flipped array.

4.12.4. See Also