为什么我得到不能通过引用错误参数2时，我使用 bindParam 与一个常量值？

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When using bindParam() you must pass in a variable, not a constant. So before that line you need to create a variable and set it to null

$myNull = null;
$stmt->bindParam(':v1', $myNull, PDO::PARAM_NULL);

You would get the same error message if you tried:

$stmt->bindParam(':v1', 5, PDO::PARAM_NULL);

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最佳答案

You need to use bindValue, not bindParam

bindParam takes a variable by reference, and doesn't pull in a value at the time of calling bindParam. I found this in a comment on the PHP docs:

bindValue(':param', null, PDO::PARAM_INT);

P.S. You may be tempted to do this bindValue(':param', null, PDO::PARAM_NULL); but it did not work for everybody (thank you Will Shaver for reporting.)

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When using INTEGER columns (that can be NULL) in MySQL, PDO has some (to me) unexpected behaviour.

If you use $stmt->execute(Array), you have to specify the literal NULL and cannot give NULL by variable reference. So this won't work:

// $val is sometimes null, but sometimes an integer
$stmt->execute(array(
':param' => $val
));
// will cause the error 'incorrect integer value' when $val == null

But this will work:

// $val again is sometimes null, but sometimes an integer
$stmt->execute(array(
':param' => isset($val) ? $val : null
));
// no errors, inserts NULL when $val == null, inserts the integer otherwise

Tried this on MySQL 5.5.15 with PHP 5.4.1

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For those who still have problems (Cannot pass parameter 2 by reference), define a variable with null value, not just pass null to PDO:

bindValue(':param', $n = null, PDO::PARAM_INT);

Hope this helps.

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If you want to insert NULL only when the value is empty or '', but insert the value when it is available.

A) Receives the form data using POST method, and calls function insert with those values.

insert( $_POST['productId'], // Will be set to NULL if empty
$_POST['productName'] ); // Will be to NULL if empty

B) Evaluates if a field was not filled up by the user, and inserts NULL if that's the case.

public function insert( $productId, $productName )
{
$sql = "INSERT INTO products (  productId, productName )
VALUES ( :productId, :productName )";


//IMPORTANT: Repace $db with your PDO instance
$query = $db->prepare($sql);


//Works with INT, FLOAT, ETC.
$query->bindValue(':productId',  !empty($productId)   ? $productId   : NULL, PDO::PARAM_INT);


//Works with strings.
$query->bindValue(':productName',!empty($productName) ? $productName : NULL, PDO::PARAM_STR);


$query->execute();
}

For instance, if the user doesn't input anything on the productName field of the form, then $productName will be SET but EMPTY. So, you need check if it is empty(), and if it is, then insert NULL.

Tested on PHP 5.5.17

Good luck,

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I had the same problem and I found this solution working with bindParam :

    bindParam(':param', $myvar = NULL, PDO::PARAM_INT);

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Try This.

$stmt->bindValue(':v1', null, PDO::PARAM_NULL); // --> insert null

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In my case I am using:

SQLite,
prepared statements with placeholders to handle unknown number of fields,
AJAX request sent by user where everything is a string and there is no such thing like NULL value and
I desperately need to insert NULLs as that does not violates foreign key constrains (acceptable value).

Suppose, now user sends with post: $_POST[field1] with value value1 which can be the empty string "" or "null" or "NULL".

First I make the statement:

$stmt = $this->dbh->prepare("INSERT INTO $table ({$sColumns}) VALUES ({$sValues})");

where {$sColumns} is sth like field1, field2, ... and {$sValues} are my placeholders ?, ?, ....

Then, I collect my $_POST data related with the column names in an array $values and replace with NULLs:

  for($i = 0; $i < \count($values); $i++)
if((\strtolower($values[$i]) == 'null') || ($values[$i] == ''))
$values[$i] = null;

Now, I can execute:

$stmt->execute($values);

and among other bypass foreign key constrains.

If on the other hand, an empty string does makes more sense then you have to check if that field is part of a foreign key or not (more complicated).

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Based on the other answers but with a little more clarity on how to actually use this solution.

If for example you have an empty string for a time value but you want to save it as a null:

  if($endtime == ""){
$db->bind(":endtime",$endtime=NULL,PDO::PARAM_STR);
}else{
$db->bind("endtime",$endtime);
}

Notice that for time values you would use PARAM_STR, as times are stored as strings.

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Several answers have given examples of what you should do. But they haven't really explained why you should do one of those things.

The bindParam method is meant to be used with something like a loop (or just repeated statements). It binds a variable reference. So something like

$stmt = $dbh->prepare('INSERT INTO t1 (v1) VALUES(:v1)');
$stmt->bindParam(':v1', $i, PDO::PARAM_INT);
for ($i = 0; $i < 10; $i++) {
$stmt->execute();
}

Would insert values 0 through 9 in a table.

That's obviously a very simple example that could be implemented in other, more efficient ways. You could have more complex logic here. But the basic idea is that you bind a reference to a variable and then you can change the value of the variable.

You can get around the need for a reference by creating a variable before calling bindParam. But in your case, you don't particularly want to bind to a variable reference. You just want to bind a value. So go ahead and do exactly that with bindValue.

You can mostly just use bindValue. But to show why both methods exist, let's rewrite the previous example to use bindValue instead of bindParam:

$stmt = $dbh->prepare('INSERT INTO t1 (v1) VALUES(:v1)');
for ($i = 0; $i < 10; $i++) {
$stmt->bindValue(':v1', $i, PDO::PARAM_INT);
$stmt->execute();
}

This will work, but you have to call bindValue on every iteration of the loop whereas you only needed to call bindParam once. But you aren't doing anything like that, so you can just

$stmt->bindValue(':v1', null, PDO::PARAM_INT);

And everything will work, as stated in the accepted answer. Because you want to bind a value, not a variable reference.