Can I stack functions on a variable?

Question

Is there a problem with stacking functions on a variable. It seems to work but is there an underlying risk or problem doing this?

$username = mysql_real_escape_string(strtolower($_POST['username']));

I don't really see a problem with it but I'm relatively new to PHP and I'm not sure if this would cause issues. As compared to:

$username = $strtolower($_POST['username']);
$username = mysql_real_escape_string($username);

Just wondering if this is the best way to do it.

Short answer: Yes. You could, of course, just run it to find out. — Jared Farrish, Jul 29 '14 at 01:33
It's perfectly valid. Unless it impairs readability, it's actually preferred over temporary variables. — mario, Jul 29 '14 at 01:33
@JaredFarrish If you notice, I know it works, I was asking if it's "okay" to do in terms of clean coding. — KriiV, Jul 29 '14 at 01:34
That completely depends on who you're asking and their style of coding. Note that this type of question would be classified as a *Close: Primarily opinion based* question, as well, if this is what you're asking. — Jared Farrish, Jul 29 '14 at 01:35
@mario - Nah man, that is the *Bedknobs and Broomsticks* of adventure programming. `:P` — Jared Farrish, Jul 29 '14 at 01:38
@mario Ah yeah, I knew that. I'm using PDO now anyway, but it was just an example. Thanks — KriiV, Jul 29 '14 at 01:39

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

There is no problem in nesting functions like mysql_real_escape_string(strtolower($value)) as you propose. It can lead to easy to read and understand code. I would caution against going overboard with it, and nesting many many functions, however, as that quickly gets unreadable.

But wait!!!

Do not be tempted to nest everything. Some functions require good error checking to be used effectively. The mysql_*() functions related to querying and fetching in particular should not be nested (actually they should not be used at all, use MySQLi or PDO instead). It is pretty safe to nest string functions as you attempted, but always check the manual at http://php.net to make sure of the possible return types of the functions you are nesting. Be especially careful with functions that return resources, such as those handling database connections, file handles, sockets, etc.

// Don't do this:
// mysqli_query() will return false on failure, which will 
// not be a valid argument to mysqli_fetch_array()
$row = mysqli_fetch_array(mysqli_query($conn, "SELECT * FROM tbl"));

If the query fails, it will result in a fatal error in your code since the object passed to the outermost function is not the type expected.

Instead, cases like this require error checking that make nesting impractical:

$result = mysqli_query($conn, "SELECT * FROM tbl");
if ($result) {
  $row = mysqli_fetch_array($result);
}

Another case to be wary of are functions which expect references as their arguments. For example, array_merge() returns an array but sort() expects a reference to an array (you wouldn't do this anyway since sort() doesn't return an array):

sort(array_merge([3,2,41], [5,1]));
// PHP Strict standards:  Only variables should be passed by reference in php shell code on line 1

A related and more likely use case in practical terms would be passing a function return value into empty() (which is technically a language construct rather than a function). Prior to PHP 5.5, only variables could be passed in, so the following would result in a parse error:

// Error in PHP < 5.5
if (empty(func_returning_string()) {}

Not a problem if you set *mysqli* to throw exceptions... `mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT)` — Phil, Jul 29 '14 at 01:44
@Phil Sure - mysqli is the closest correlate example to the OP's, but this applies to many resource-returning things. I'll add a note. — Michael Berkowski, Jul 29 '14 at 01:45

score 1 · Answer 2 · answered Jul 29 '14 at 01:34

1

Yes, stack as many function calls as you like. Your example won't work however because you're missing a right paren

answered Jul 29 '14 at 01:34

user1433150

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Can I stack functions on a variable?

2 Answers2

But wait!!!