Calling a function within the parameter of while loop in PHP

Okay here is simple test for you,

let have an array which has value which is null as demonstrated below,

$row = array('value' => null);

and now lets check for that using if condition

if($row)
{
    echo "null is making variable value to true so condition will work.";
}

paste code and run it i am sure that you will see message inside of if condition.

Your Condition

You are using $result->fetch_assoc() so as you know it will return array which might have null value as demonstrated in above example.

but as you see it will return true for the $result because $result actually has value assigned and it is true.

so condition will be satisfied.

In short the while loop condition is looking for true:

while(true){
//do this
}

So until this expression $row = $result->fetch_assoc() resolves to true the while loop will continue.

The question is when it will be not true? when all rows retrieved.

how does this condition evaluate to true if $row is null?

It doesn't. That's the point.

Given your code:

while($row = $result->fetch_assoc()){
    echo $row['data'];
}

The loop condition can be rewritten to the following equivalent:

while (($row = $result->fetch_assoc()) != false) {
    echo $row['data'];
}

In other words, while $row is not falsy, it will continue the loop; null is also considered falsy.

the following explanation to your code may help you

// run the query. this returns a mysqli_result object on success.
// on failure query function returns false and you have to handle this condition.
$result = $mysqli->query("SELECT col FROM table");

// fetch_assoc() here you fetch one row at a time from the mysqli_result
// object into $row. this also moves the pointer to the next row
// so the next call returns the next row.
// This returns false when there is no more rows and exit your while loop
while($row = $result->fetch_assoc()){
    echo $row['data'];
}

links you may want to refer
http://www.php.net/manual/en/class.mysqli-result.php
http://php.net/manual/en/mysqli-result.fetch-assoc.php