According to the php manual.

Simplifying: One is a return and one value, another is to create a Generator.

YIELD

  

A yield statement looks very much like a return, except that instead of stopping the execution of the function and returning, yield yields a value for the loop code over the generator and pauses the execution of the generator function.

I'm not going to talk about Generators here, because this question explains it very well:

What are the advantages to use a Generator (yield) in PHP?

So when you use yield you are indicating that the value passed to it will be part of a desired iteration step. There will be a "pause" in the iteration and those values will be returned in an object of class Generator .

Generator in turn implements an interface called Iterator in PHP. It would be good to understand its usage, because in this case, it would not be important to learn only the Generator syntax before learning what iterator is.

If you know what a Iterator is, then we can say: When you call a Generator object with foreach , you are invoking the methods in the following sequence:

foreach (gen() as $key => $value) {

}

Equivalent to:

$gen = gen();

$gen->rewind();

while ($gen->valid()) {
    $key  = $gen->key();
    $value = $gen->current();

   $gen->next();
}

RETURN

The return , which translates as "return", serves to return a value for a function. There are two different cases, since in the first case, Generator is just a "pause" for the loop that was proposed therein. Already return represents the definite value returned for that function call.

Can I use the two together?

Depends on the version of PHP you are using. If you are using versions prior to PHP 7, you will get an exception when trying to set return and yield to the same function.

In PHP 7, you can do this. In this case, you will have to use the Generator::getReturn method.

function gen()
{
     yield 1 => 2;
     yield 3 => 4;

    return 5;
}

gen()->getReturn(); // 5

Other differences

The return can be used outside a function, since yield can not.

The best way to get a generator through yield , having the Generator object directly in a variable, is only with the self invoking function. But it's only available for PHP 7.

Example:

 $gen = (function ()
 {
    yield "A" => "B";

  })();

 var_dump($gen); // Generator(object);

The yield can be processed according to the number of declarations, return not. You can only have more than one return in the function, but when the first one is executed, the rest will be ignored. yield is not like this.

function hello()
{
    yield "Stack Overlow";
    yield "Mandioca";
    yield "Milho";
}

$gen = hello();
var_dump($gen->current());
$gen->next();
var_dump($gen->current());

The output will be:

string(13) "Stack Overlow"
string(8) "Mandioca"

The answer from Wallace Maxters is already good, I'm just going to give a slightly different view of the same thing.

I see yield as the generator / iterator quoted. It is responsible for maintaining state between the calls of the function that the use. It is responsible for storing where it stopped in the execution of the function, and hence where it should resume when it is run again.

As a matter of fact, it has an implicit%% of its execution. Then it is usually used instead of return pure which is the simple return without saving state.

Additional information for those who are arriving here with total doubt on the subject:

• How useful is the "yield" reserved word?
• What are the advantages of using a Generator (yield) in PHP?
• With an iterator / generator in javascript?
• yield does not return data
• How to read / translate the keyword yield?

A simple explanation is that the return returns the value and ends the function, the yield returns the value and gives a "pause" in the function (on the next call it continues soon after the yield).

In practice one is not compatible with the other and they are used differently in different contexts. If you are interested, look for generating functions or iterators. If not, just use return and ignore yield.