> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # PHP Yield From Expression The `yield from` expression delegates the execution of a generator to another iterable structure, such as an array, a `Traversable` object, or another generator. It allows a parent generator to yield all values from a sub-iterable directly to the calling context without requiring an explicit `foreach` loop to extract and re-yield those values. ```php theme={"dark"} yield from ; ``` ## Execution Mechanics When the PHP engine evaluates a `yield from` expression, it temporarily suspends the execution of the current generator and transfers control to the provided ``. 1. **Value Emission:** Every value and key produced by the sub-iterable is passed directly to the outer generator's caller. 2. **Key Preservation:** `yield from` strictly preserves the keys yielded by the delegated iterable. If an array with specific string or integer keys is passed to `yield from`, those exact keys are emitted to the caller. 3. **Bidirectional Communication:** If the delegated iterable is another generator, `yield from` establishes a transparent conduit. Calls to `Generator::send()` or `Generator::throw()` on the parent generator are forwarded directly to the active sub-generator. ## Expression Evaluation and Return Values Both `yield` and `yield from` are expressions. However, they evaluate to different values based on their mechanics. While a standard `yield` expression evaluates to the value passed back to the generator via `Generator::send()`, a `yield from` expression evaluates to the return value of the delegated iterable once it is completely exhausted. * **Generators:** If the delegated iterable is a generator that terminates with a `return` statement, the `yield from` expression evaluates to that returned value. * **Arrays and Traversables:** If the delegated iterable is an array or a standard `Traversable` object, the `yield from` expression evaluates to `null`. ```php theme={"dark"} function subGenerator(): Generator { yield 'A'; yield 'B'; return 'Sub-generator execution finished'; } function parentGenerator(): Generator { // Delegates yielding to an array. Evaluates to null. yield from [1, 2]; // Delegates yielding to another generator. Evaluates to the generator's return value. $returnValue = yield from subGenerator(); yield $returnValue; } // Iteration output: 1, 2, 'A', 'B', 'Sub-generator execution finished' ``` ## Syntax Rules * `yield from` can only be used within a function, method, or closure, which implicitly marks that callable as a `Generator`. * It accepts exactly one expression that must evaluate to an `iterable` type. Passing a non-iterable scalar or object will result in a fatal `Error` with the message *"Can use 'yield from' only with arrays and Traversables"*. * Because it is an expression, its result can be assigned directly to a variable (e.g., `$result = yield from $iterable;`). Parentheses are not required for simple assignment, but they are strictly required when the `yield from` expression is evaluated within larger operations (e.g., `$result = 5 + (yield from $iterable);`).

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