Generator

In PHP the foreach-loop is powerful and leads to clean, concise code. But for complex uses, foreach is harder to use—a generator can address this problem.

With the yield-statement, a generator function can return multiple times (it pauses its execution). This allows a foreach-statement to skip over elements, or perform other computations.

We introduce a generator function in the PHP program here called filter_by. It receives an array and a required starting letter. It filters out elements that do not have the starting letter.

Step 1 We create an array of 4 strings. Please note that only 2 of the strings starts with the letter "b."

Step 2 We use a foreach-loop and specify our generator function (filter_by) in the first part of the foreach statement.

Step 3 In the generator function, we loop over the array argument (source) with a foreach-loop.

Step 4 If the first character of the string element matches the letter argument, we yield that item.

Step 5 We print out the strings that start with the letter "b" inside the foreach-loop here.

function filter_by($source, $letter) {
    // Step 3: Loop over array elements.
    foreach ($source as $item) {
        // Step 4: If first letter of element matches, yield it.
        if ($item[0] == $letter) {
            yield $item;
        }
    }
}

// Step 1: create an array.
$values = ["bird", "?", "cat", "barn"];

// Step 2: call generator on the array.
foreach (filter_by($values, "b") as $item) {
    // Step 5: Print the items yield ed from the generator.
    var_dump($item);
}
string(4) "bird"
string(4) "barn"

Generators can move logic from the body of a foreach-loop into a reusable function. This can clean up PHP programs by reducing duplicated statements.