PHP PDO with foreach and fetch PDOStatement是怎樣被Foreach直接遍歷的

Anyone know why does this happen? I'm just a PHP beginner, thanks for your help!

The following codes:

<?php
try {
    $dbh = new PDO("mysql:host=$hostname;dbname=$dbname", $username, $password);
    echo "Connection is successful!<br/>";
    $sql = "SELECT * FROM users";
    $users = $dbh->query($sql);
    foreach ($users as $row) {
        print $row["name"] . "-" . $row["sex"] ."<br/>";
    }
    foreach ($users as $row) {
        print $row["name"] . "-" . $row["sex"] ."<br/>";
    }
    $dbh = null;
}
catch (PDOexception $e) {
    echo "Error is: " . $e-> etmessage();
}

OUTPUT:

Connection is successful!

person A-male
person B-female

Running "foreach" twice is not my purpose, I'm just curious why TWO "foreach" statements only output the result once?

Following is the similar case:

<?php
try {
    $dbh = new PDO("mysql:host=$hostname;dbname=$dbname", $username, $password);
    echo "Connection is successful!<br/>";
    $sql = "SELECT * FROM users";
    $users = $dbh->query($sql);
    foreach ($users as $row) {
        print $row["name"] . "-" . $row["sex"] ."<br/>";
    }
    echo "<br/>";
    $result = $users->fetch(PDO::FETCH_ASSOC);
    foreach($result as $key => $value) {
        echo $key . "-" . $value . "<br/>";
    }
    $dbh = null;
}
catch (PDOexception $e) {
    echo "Error is: " . $e-> etmessage();
}

OUTPUT:

Connection is successful!

person A-male
person B-female

SCREAM: Error suppression ignored for
Warning: Invalid argument supplied for foreach()

But when I delete the first "foreach" from the above codes, the output will become normal:

<?php
try {
    $dbh = new PDO("mysql:host=$hostname;dbname=$dbname", $username, $password);
    echo "Connection is successful!<br/>";
    $sql = "SELECT * FROM users";
    $users = $dbh->query($sql);

    echo "<br/>";
    $result = $users->fetch(PDO::FETCH_ASSOC);
    foreach($result as $key => $value) {
        echo $key . "-" . $value . "<br/>";
    }
    $dbh = null;
}
catch (PDOexception $e) {
    echo "Error is: " . $e-> etmessage();
}

OUTPUT:

Connection is successful!

user_id-0000000001
name-person A
sex-male

ANSWERS:

FIRST ANSWER:

A PDOStatement (which you have in $users) is a forward-cursor. That means, once consumed (the first foreach iteration), it won't rewind to the beginning of the resultset.

You can close the cursor after the foreach and execute the statement again:

$users       = $dbh->query($sql);
foreach ($users as $row) {
    print $row["name"] . "-" . $row["sex"] ."<br/>";
}

$users->execute();

foreach ($users as $row) {
    print $row["name"] . "-" . $row["sex"] ."<br/>";
}

Or you could cache using tailored CachingIterator with a fullcache:

$users       = $dbh->query($sql);

$usersCached = new CachedPDOStatement($users);

foreach ($usersCached as $row) {
    print $row["name"] . "-" . $row["sex"] ."<br/>";
}
foreach ($usersCached as $row) {
    print $row["name"] . "-" . $row["sex"] ."<br/>";
}

You find the CachedPDOStatement class as a gist. The caching itertor is probably more sane than storing the resultset into an array because it still offers all properties and methods of the PDOStatement object it has wrapped.

thanks for mentioning "forward-cursor", I will google it. – nut Mar 13 '13 at 13:23
Thanks, although I can't understand it clearly but your example is definitely useful to me in the future. – nutMar 13 '13 at 13:42
Please see yet another edit, I had a typo: Use $users->execute(); after the first foreach to reset the cursor to the beginning of the result-set. This is most likely what you're looking for. The cache is a more specific solution and not really necessary as you can do $users->execute();. I added it just for the general example. – hakre Mar 13 '13 at 13:45
thank you, the first one seems to be clearer to me, I will look into the execute function in php manual. – nutMar 13 '13 at 13:51
Yes, it's much more straight forward. The method is here: php.net/manual/en/pdostatement.execute.php – hakre Mar 13 '13 at 13:53

SECOND ANSWER:

foreach over a statement is just a syntax sugar for the regular one-way fetch() loop. If you want to loop over your data more than once, select it as a regular array first

$sql = "SELECT * FROM users";
$stm = $dbh->query($sql);
// here you go:
$users = $stm->fetchAll();

foreach ($users as $row) {
    print $row["name"] . "-" . $row["sex"] ."<br/>";
}
echo "<br/>";
foreach ($users as $row) {
    print $row["name"] . "-" . $row["sex"] ."<br/>";
}

Also quit that try..catch thing. Don't use it, but set proper error reporting for PHP and PDO

Thank you for the tip, learning the prepare and execute is my next step. – nut Mar 13 '13 at 13:38
4

@YourCommonSense Why do you have two for each loops in your answer? I'm not fault finding, just learning... – Norman Oct 14 '13 at 15:14
3

using pdo->query() is definitely NOT a wrong way of using the library. exec() and query() are there to be used. The only wrong usage was trying to go through the statement two times, he should just have fetchAll() and then he can iterate several times through the array.. If you stay to your word that using query() is wrong, give a proper reason please. – John Sep 24 '16 at 23:08
I too am unclear on what is wrong with using query(), as mentioned in your tutorial. Is there something about this particular implementation that's not ideal? – showdev Apr 21 '17 at 0:40
1

@showdev you are right. I don't remember the context o this answer but definitely it looked alien, so I edited it– Your Common Sense Apr 21 '17 at 3:35

This is because you are reading a cursor not an array. This means that you are reading sequentially through the results and when you get to the end you would need to reset the cursor to the beginning of the results to read them again.

If you did want to read over the results multiple times, you could use fetchAll (http://www.php.net/manual/en/pdostatement.fetchall.php) to read the results into a true array and then it would work as you are expecting.

$users = $dbh->query($sql);
foreach ($users as $row) {
    print $row["name"] . "-" . $row["sex"] ."<br/>";
}
foreach ($users as $row) {
    print $row["name"] . "-" . $row["sex"] ."<br/>";
}

Here $users is a PDOStatement object over which you can iterate. The first iteration outputs all results, the second does nothing since you can only iterate over the result once. That's because the data is being streamed from the database and iterating over the result with foreach is essentially shorthand for:

while ($row = $users->fetch()) ...

Once you've completed that loop, you need to reset the cursor on the database side before you can loop over it again.

$users = $dbh->query($sql);
foreach ($users as $row) {
    print $row["name"] . "-" . $row["sex"] ."<br/>";
}
echo "<br/>";
$result = $users->fetch(PDO::FETCH_ASSOC);
foreach($result as $key => $value) {
    echo $key . "-" . $value . "<br/>";
}

Here all results are being output by the first loop. The call to fetch will return false, since you have already exhausted the result set (see above), so you get an error trying to loop over false.

In the last example you are simply fetching the first result row and are looping over it.

PHP PDO with foreach and fetch PDOStatement是怎樣被Foreach直接遍歷的

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