php PDO fetchAll() - while not working, foreach works

Question

I would like to know if i'm doing fine OR fetchAll() doesn't work with WHILE.

here is an exemple

$db=new PDO("mysql:host=" .$dbhost. "; dbname=" . $dbname, $dbuser, $dbpass);

$page=$db->prepare("SELECT * FROM page");
$page->execute();

foreach ($page->fetchAll(PDO::FETCH_ASSOC) as $row) {

//echo a row
//is working
}

however, i if try looping with a while

while ($row=$page->fetchAll(PDO::FETCH_ASSOC)){

//echo a row
//Show empty
}

i tryed to use only fetch(), it was working, my question: why fetchAll() doesn't work with "WHILE" ?

Answer 1

Fetch all returns all of the records remaining in the result set. With this in mind your foreach is able to iterate over the result set as expected.

For the equivalent while implementation should use $page->fetch(PDO::FETCH_ASSOC);

while ($row = $page->fetch(PDO::FETCH_ASSOC)){
   // do something awesome with row
} 

if you want to use a while and fetch all you can do

$rows = $page->fetchAll(PDO::FETCH_ASSOC);

// use array_shift to free up the memory associated with the record as we deal with it
while($row = array_shift($rows)){
   // do something awesome with row
}

A word of warning though: fetch all will do exactly that, if the result size is large it will stress the resources on your machine. I would only do this if I know that the result set will be small, or I'm forcing that by applying a limit to the query.