PHP yield

The PHP yield keyword is used to create a generator function. It is a built-in keyword in the PHP. A generator function is the same as the normal function, except returning a single value, the generator yield keyword returns many values as required to return from the generator function. Any function containing yield is a generator function. The Generator function same as the iterator, which loop through with a foreach loop, and each time it needs a value, the generator yield a value; the yield keyword is the same as the return keyword, except that instead of stopping execution.

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Syntax

yield $value;

Parameters –

value – This is not optional, that specifies the value which is to be returned.

Return value – The return value of this method is the values required to be returned in the generating function.

Working of the PHP yield keyword

The PHP yield keyword uses in the generator function to returns or yields the multiple values. When the generator function is called, it returns an iterator object which can be iterated. When we iterate this object by a foreach loop, which calls the object’s iteration methods by PHP each time it needs a value, then the state of the generator function is saved when the generator function yields a value that is resumed to get the next value. When there are no values to yield, then the generator simply exits, and the calling code can continue just as the array run out of values.

Examples of the PHP yield keyword

Different examples are mentioned below:

Example #1 – yielding multiple values

Next, we write the PHP code to understand the PHP yield keyword more clearly with the following example, where the yield keyword is used to yielding multiple values from the generator function, as below –

Code:

<?php // creating function to generate even numbers in between 2 to 10
function gen_even_numbers() {
for ($i = 2; $i <= 10; $i+=2) {
// Note that $i is preserved between yields.
yield $i;
}
}
// calling the gen_even_numbers function
$generator = gen_even_numbers();
// iterating over the yeilded values of the gen_even_numbers function
print( " The yeilded values of the generator function are : \n ");
foreach ($generator as $value) {
print( " $value\n ");
}
?>

Output:

As in the above program, the gen_even_numbers() function is created to find the even numbers between 2 to 10 and yield or return when it is calling iteratively. In each iteration, it returns the values one by one and which are displaying as well, as we can see in the output.

Example #2 – iterator_to_array() method

Next, we write the PHP code to understand the PHP yield keyword, where the yield keyword is used to yielding multiple values from the generator function, and that generator function is calling inside another generator function, as below –

Code:

<?php // creating function to generate even numbers in between 2 to 10
function gen_even_numbers() {
for ($i = 2; $i <= 10; $i+=2) {
// Note that $i is preserved between yields.
yield $i;
}
}
// creating function to call gen_even_numbers function
function call_gen()
{
yield 2;
yield from gen_even_numbers();
yield 10;
}
// calling the call_gen function
$generator = call_gen();
// iterating over the yeilded values of the call_gen function
print( " The yeilded values of the generator function by foreach loops are : \n ");
foreach ($generator as $value) {
print( " $value\n ");
}
print("<br>");
print( "The yeilded values of the generator function by iterator_to_array are : \n ");
var_dump(iterator_to_array(call_gen()));
?>

Output:

As in the above program, the gen_even_numbers() function is created to find and return the even numbers in between 2 to 10, and then another function call_gen() is used to call gen_even_numbers() function by using yield from, which does not reset the keys. Later in the code, the iteration is performing two ways to. The first way is using the foreach loop, which is iterating over the values in sequence as they are yielding in the call_gen() function, so keys are not reset and getting output as 2 2 4 6 8 10 10. Where as the second way is using the iterator_to_array() method, which returns a keyed array by default and possibly may lead to unexpected results, which we can see in the output.

Example #3 – inside the reference generator function

Next, we write the PHP code to understand the PHP yield keyword, where the yield keyword is used inside the reference generator function, as below –

Code:

<?php // creating function to generate even numbers in between 2 to 10
function &gen_even_numbers() {
$i = 2;
while($i <= 10) {
// yield does not come out from the loop or function
yield $i;
$i+=2;
}
}
// calling the gen_even_numbers function
$generator = gen_even_numbers();
// iterating over the yeilded values of the call_gen function
print( " The yeilded values of the reference generator function are : \n ");
foreach ($generator as &$value) {
print( " $value\n ");
}
?>

Output:

In the above program, the gen_even_numbers() function is a reference function as it is prefixed by the “&”; it is used to find and return the even numbers in between 2 to 10. Farther iterating over the return object of this function, and each time the iterated value is storing into a variable which is again prefixed by the “&”.

Conclusion

The PHP yield keyword is a built-in keyword used to create a generator function or return a.

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