Why out of several str_replace() functions only the last one affects the string?

Question

I have set up a very simple code snippet:

$string = 'Some random words. Some more random, very random words.';
$words = explode(" ", $string);

for ($i = 0; $i < count($words); $i++) {
    $word = $words[$i];
    $words[$i] = str_replace(".", "!", $word);
    $words[$i] = str_replace(",", "?", $word);
}

print_r($words);

The output is this:

Array
(
    [0] => Some
    [1] => random
    [2] => words.
    [3] => Some
    [4] => more
    [5] => random?
    [6] => very
    [7] => random
    [8] => words.
)

Why only the second str_replace() function affect the string? If I remove the second str_replace() the first one works perfectly. It's not about usage of str_replace() but I believe me doing something very very simply wrong.

By the way - I am aware of preg_replace() and passing an array to str_replace() but would like to hear about this particular situation :).

EDIT: Thank you all for blazing quick responses. I am in a shame for such an issue but it really didn't catch my eyes at first. Thanks everyone! I will accept the first correct answer by Mike Brant .

Answer 1

It is because your second statement uses $word as the subject of replacement and not $words[$i] which was where you assigned the string after the first replacement.

You can fix by either working directly with $words[$i] the entire time, or working exclusively with your temp variable and then making assignment like this:

for ($i = 0; $i < count($words); $i++) {
    $word = $words[$i];
    $word = str_replace(".", "!", $word);
    $words[$i] = str_replace(",", "?", $word);
}

Answer 2

Because you were applying str_replace to the same string over and over again. You need to reassign the new string (with replaced characters) and perform another replace on the updated value

for ($i = 0; $i < count($words); $i++) {
    $word = $words[$i];  //initial value
    $word = str_replace(".", "!", $word);  //change $word to modified text
    $word = str_replace(",", "?", $word);  //change $word to modified text
    $words[$i] = $word;
}

The $word value doesn't get changed by str_replace , so it remains the same. To change it, you need to assign the return value/result from str_replace

Answer 3

Change to:

for ($i = 0; $i < count($words); $i++) {        
    $words[$i] = str_replace(".", "!", $words[$i]);
    $words[$i] = str_replace(",", "?", $words[$i]);
}

Even shorter:

for ($i = 0; $i < count($words); $i++) {        
    $words[$i] = str_replace(array(".", ","), array("!", "?"), $words[$i]);        
}