Fastest way to iterate array in PHP
From these options C would be the obvious answer.
The remaining options (besides A) may be used to achieve that, depending on the code inside the parenthesis, but the question does NOT show that code. So it must be C.
And you are answering the wrong question - yes doing count() before the for cycle will improve performance, but this question is not about performance.
You can iterate and modify every element of an array with any of the shown constructs. But some notes on that:
b) Is only useful if the array is a numeric array with the keys from 0 to n-1.
c) Is useful for both kinds of arrays. Additionally $value is a reference of the element’s value. So changing $value inside foreach will also change the original value.
d) Like c) except $value is a copy of the value (note that foreach operates on a copy of $array). But with the key of the element you can access and change the original value with $array[$key].
e) Like d). Use $array[$key] to access and change the original element.