How does looping though a PHP array using While() work? - php

Question

I have found some PHP code that connects to a MySQL database and gets the column CityName for each row of the table Cities. I'm curious why while() loop is used and not for() or foreach. So here are my questions regarding how while() works in case of looping through arrays:
First, isn't $row variable an 2D array which it's rows contains the list of cities from the SQL query and it's columns contain the columns of each row of the query?
If this is the case, couldn't for() or foreach() be used to loop through the $row array?
Second, how does while() know when the array ends using only $row = $stmt->fetch_assoc() in the while()'s first line in order to end the loop?
Third, how does while() move to the next row of the $row array without using next() at the end of the loop?
And last but not least, how does echo $row['CityName']; output the city name of each row of the $row array without specifying the row of the array to use but only it's column CityName?
Thanks for any answers.
$query="SELECT CityName FROM Cities";
if($stmt = $connection->query("$query"))
{
while ($row = $stmt->fetch_assoc())
{
echo $row['CityName'];
}
}
else
{
echo $connection->error;
}

Answer 1

You could loop through $row, because it is an array (a simple array, not a 2d array); but you're not looping through an array called $row with the while, you're iterating over the resultset returned by $stmt->fetch_assoc() - which isn't an array- and assigning the value of a single returned row to $row in that statement (note the = for assignment) from the resultset.
while itself doesn't magically move any pointer; it's the call to $stmt->fetch_assoc() that not only returns a single row result, but moves the resultset pointer to the next result (and determines when it has reached the end of the resultset)

Answer 2

while($this_is_true){
// do this
}
# if the condition is not true for this while stmt, it will not execute (not even once)
do{
// stuff here
} while($thisIsTrue);
# even if the condition isn't true, a do-while executes at least once (the first time)
The point is that they are using a while() because it's a lazy way to say "only do it if it's true", so they don't have to check if it's true before they loop through it. WIth a for loop, you need to know the number of results you are looping through. For a do, you need to know that there are results before you attempt to use the results (echo). So, you use a while() to check if it's valid and then execute it, with just that piece of code.
Personally, I like to do...
if($query->num_rows > 0){
$query->bind_result($bindvar);
while($query->fetch()){
// do stuff
}
$query->close();
} else {
// no results found
}

Answer 3

You can use foreach instead of while. At least if stmt is a mysqli_result and you PHP version is not terribly outdated (newer than 5.4). In that case your loop would be:
foreach($stmt as $row)
{
echo $row['CityName'];
}
But this does not mean that $stmt is a 2D array. It's an object that implements the Traversable interface. That means that the foreach loop will implicitly invoke the required methods.