Question

I am a dead beginner with PHP and have been reading through ‘PHP for the Web: Visual Quickstart Guide 4th Ed.’ by Larry Ullman and have a question regarding something I came across in the book.

At the end of each chapter he has a few questions for review and I am stuck on one of them and not sure if I have the correct answer or the correct train of though regarding it.

The question is as follows:

Without knowing anything about $var will the following conditional be TRUE or FALSE? Why?

if ($var = 'donut') {...

I am apt to say that it will be false because we don’t know if $var has been assigned the value donut yet within the program but I am not sure.

Can anyone help explain this to me so I can grasp this concept and feel confident about it?

Answer 1

This conditional will always evaluate to TRUE because the value donut is assigned, and then the value of $var is returned to the if() statement. The assignment happens first.

A successful assignment to a variable causes that variable to be returned immediately. A non-empty string is a "truthy" value, and is returned as such.

If instead it was assigned as:

if ($var = "") {}

It would evaluate to FALSE, according to PHP’s boolean evaluation rules:

var_dump((bool) "");        // bool(false)
var_dump((bool) 1);         // bool(true)
var_dump((bool) -2);        // bool(true)
var_dump((bool) "foo");     // bool(true)
var_dump((bool) 2.3e5);     // bool(true)
var_dump((bool) array(12)); // bool(true)
var_dump((bool) array());   // bool(false)
var_dump((bool) "false");   // bool(true)

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Addendum

Just to add, as a practical example of assignment inside a flow control conditional you probably see almost every day — the while() loop we typically use to retrieve a rowset from a MySQL result resource:

while ($row = mysql_fetch_assoc($result)) {
  // executes this inner block as long as $row doesn't 
  // recieve a FALSE assignment from mysql_fetch_assoc()
  // reaching the end of its rowset
}