Is it better call a function every time or store that value in a new variable?

TLDR: it's better to set a variable, calling sizeof() only once. (IMO)

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I ran some tests on the looping aspect of this small array:

$myArray = array("bill", "dave", "alex", "tom", "fred", "smith", "etc", "etc", "etc");

// A)
for($i=0; $i<10000; $i++) {
  echo sizeof($myArray);
}

// B)
$sizeof = sizeof($myArray);
for($i=0; $i<10000; $i++) {
  echo $sizeof;
}

With an array of 9 items:

A) took 0.0085 seconds
B) took 0.0049 seconds

With a array of 180 items:

A) took 0.0078 seconds
B) took 0.0043 seconds

With a array of 3600 items:

A) took 0.5-0.6 seconds
B) took 0.35-0.5 seconds

Although there isn't much of a difference, you can see that as the array grows, the difference becomes more and more. I think this has made me re-think my opinion, and say that from now on, I'll be setting the variable pre-loop.

Storing a PHP integer takes 68 bytes of memory. This is a small enough amount, that I think I'd rather worry about processing time than memory space.

In general, it is preferable to assign the result of a function you are likely to repeat to a variable.

In the example you suggested, the difference in processing code produced by this approach and the alternative (repeatedly calling the function) would be insignificant. However, where the function in question is more complex it would be better to avoid executing it repeatedly.

For example:

for($i=0; $i<10000; $i++) {
  echo date('Y-m-d');
}

Executes in 0.225273 seconds on my server, while:

$date = date('Y-m-d');

for($i=0; $i<10000; $i++) {
  echo $date;
}

executes in 0.134742 seconds. I know these snippets aren't quite equivalent, but you get the idea. Over many page loads by many users over many months or years, even a difference of this size can be significant. If we were to use some complex function, serious scalability issues could be introduced.

A main advantage of not assigning a return value to a variable is that you need one less line of code. In PHP, we can commonly do our assignment at the same time as invoking our function:

$sql = "SELECT...";

if(!$query = mysql_query($sql))...

...although this is sometimes discouraged for readability reasons.

In my view for the sake of consistency assigning return values to variables is broadly the better approach, even when performing simple functions.