PHP foreeach deleting array element

Question

Today while working on text analysing tool for blogs, I found PHP behavior very strange for me and just couldn't wrap my head around it. While normalizing text, I was trying to remove words below minimum length, so I wrote this in my normalization method:

if ($this->minimumLength > 1) {
    foreach ($string as &$word)
    {
        if (strlen($word) < $this->minimumLength) {
            unset($word);
        }
    }
}

Strangely, this would leave some words below allowed length in my array. After searching my whole class for mistakes, I gave a shot at this:

if ($this->minimumLength > 1) {
        foreach ($string as $key => $word)
        {
            if (strlen($word) < $this->minimumLength) {
                unset($string[$key]);
            }
        }
    }

And voila! This worked perfectly. Now, why would this happen ? I checked out PHP Documentation and it states:

If a variable that is PASSED BY REFERENCE is unset() inside of a function, only the local variable is destroyed. The variable in the calling environment will retain the same value as before unset() was called.

Does foreach act here as a calling environment because it has it's own scope?

Answer 1

No, there is no function call here and no variable is being passed by reference (you are simply capturing by reference during the iteration).

When you iterate by reference the iteration variable is an alias to the original. When you use this alias to refer to the original and modify its value the change will remain visible in the array being iterated.

However, when you unset the alias the original variable is not "destroyed"; the alias is simply removed from the symbol table.

foreach ($string as $key => &$word)
{
    // This does not mean that the word is removed from $string
    unset($word);

    // It simply means that you cannot refer to the iteration variable using
    // $word from this point on. If you have captured the key then you can
    // still refer to it with $string[$key]; otherwise, you have lost all handles
    // to it for the remainder of the loop body
}

Answer 2

When you were calling unset($word) inside your if statement, you were removing the $word variable itself, without making any changes to the array $string.