Assign variable value in php

Question

i'm just confused about assigning variable values on php to another one , how does it work?! for Example:

 <?PHP
    $var1=mysqli_fetch_array($query);
    while($var2=$var1)
     {
       echo $var2[$key];   /** this wont't work Correctly however $var1 it's 
       value = to mysqli_fetch_array**/
     }

    while($var1=mysqli_fetch_array($query))
     {
      echo $var1[$key];   /** this will work , why ! **/
     }

 ?>

Answer 1

A peculiarity in PHP's assignment behaviour, is that it also returns the result of the assigned value. This allows for statements such as:

$a = $b = $c = 3; // All of a, b and c will equal 3

And:

while ($variable = call_a_function()) {
  do_something_with($variable);
}

In the latter example, variable gets assigned to the output of call_a_function() at the beginning of the loop iteration; as soon as call_a_function() returns a value that evaluates to false , the loop ends. If the returned value does not evaluate to false , variable will contain whatever value was returned, until it gets overwritten again.

Your examples use a similar behaviour. The crucial difference between

$var1=mysqli_fetch_array($query);
while($var2=$var1)
{
  echo $var2[$key];
}

And:

while($var1=mysqli_fetch_array($query))
{
  echo $var1[$key];
}

...is that in the first example, $var1 is only assigned to the return value of mysqli_fetch_array($query) before the loop starts, while in the second example, $var1 is assigned to the return value of mysqli_fetch_array($query) in every iteration of the loop .

What makes the two pieces of code crucially different, in the end, is the fact that mysqli_fetch_array($query) returns different results, depending on circumstances.

Combining the code snippets into an example that works as intended, yet uses $var2, yields:

while($var2=$var1=mysqli_fetch_array($query))
{
  echo $var2[$key];
}

or

$var1=mysqli_fetch_array($query); // ask first time
while($var2=$var1)
{
  echo $var2[$key];
  $var1=mysqli_fetch_array($query); // ask again, because the answer changed
}

TL;DR: The first example asks a question once, the second asks a question many times. In this case, the intended behaviour of your code requires the question to be asked multiple times, because the answer changes over time.