I know this has been said before but I'll write a note on it too because I think it's important to keep in mind:

If you use PDO bindParam to do a search with a LIKE condition you cannot put the percentages and quotes to the param placeholder '%:keyword%'.

This is WRONG:
"SELECT * FROM `users` WHERE `firstname` LIKE '%:keyword%'";

The CORRECT solution is to leave clean the placeholder like this:
"SELECT * FROM `users` WHERE `firstname` LIKE :keyword";

And then add the percentages to the php variable where you store the keyword:
$keyword = "%".$keyword."%";

And finally the quotes will be automatically added by PDO when executing the query so you don't have to worry about them.

So the full example would be:
<?php
// Get the keyword from query string
$keyword = $_GET['keyword'];
// Prepare the command
$sth = $dbh->prepare('SELECT * FROM `users` WHERE `firstname` LIKE :keyword');
// Put the percentage sing on the keyword
$keyword = "%".$keyword."%";
// Bind the parameter
$sth->bindParam(':keyword', $keyword, PDO::PARAM_STR);
?>

This works ($val by reference):
<?php
foreach ($params as $key => &$val) {
    $sth->bindParam($key, $val);
}
?>

This will fail ($val by value, because bindParam needs &$variable):
<?php
foreach ($params as $key => $val) {
    $sth->bindParam($key, $val);
}
?>

Note that when using PDOStatement::bindParam an integer is changed to a string value upon PDOStatement::execute(). (Tested with MySQL). 

This can cause problems when trying to compare values using the === operator.

Example:
<?php
$active = 1;
var_dump($active);
$ps->bindParam(":active", $active, PDO::PARAM_INT);
var_dump($active);
$ps->execute();
var_dump($active);
if ($active === 1) {
    // do something here
    // note: this will fail since $active is now "1"
}
?>

results in:
int(1) 
int(1) 
string(1) "1"

PHP 8.1 changed the way how this method works.

Now, the fourth method must not be null. It could be 0 (for an int) but it could not be null, otherwise, it will raise a deprecated warning.

old:
$sth->bindParam('calories', $calories, PDO::PARAM_INT);
new:
$sth->bindParam('calories', $calories, PDO::PARAM_INT,0);

There seems to be some confusion about whether you can bind a single value to multiple identical placeholders. For example:

$sql = "SELECT * FROM user WHERE is_admin = :myValue AND is_deleted = :myValue ";

$params = array("myValue" => "0");

Some users have reported that attempting to bind a single parameter to multiple placeholders yields a parameter mismatch error in PHP version 5.2.0 and earlier. Starting with version 5.2.1, however, this seems to work just fine.

For details, see bug report 40417:
http://bugs.php.net/bug.php?id=40417

http://technet.microsoft.com/en-us/library/ff628166(v=sql.105).aspx

When binding null data to server columns of type varbinary, binary, or varbinary(max) you should specify binary encoding (PDO::SQLSRV_ENCODING_BINARY) using the $driver_options. See Constants for more information about encoding constants.
Support for PDO was added in version 2.0 of the Microsoft Drivers for PHP for SQL Server.

 <?php
$db = new PDO('sqlsrv:server=SQLSERVERNAME;Database=own_exchange', 'user', 'password');
$sql = "INSERT INTO dbo.files(file_name, file_source) VALUES(:file_name, :file_source)";
$stmt = $db->prepare($sql);
$stmt->bindParam(":file_name", $files->name, PDO::PARAM_STR);
$stmt->bindParam(":file_source", file_get_contents($files->tempName), PDO::PARAM_LOB, 0, PDO::SQLSRV_ENCODING_BINARY);
$stmt->execute();
?>

SQL Server 2008 R2

If this was in the documentation, I didn't stumble across it. When using bound output parameters with a stored procedure, the output parameters are updated AFTER the LAST rowset has been processed.

If your stored procedure does not return any rowsets (no SELECT statements) then you are set, your output parameters will be ready as soon as the stored procedure is processed.

Otherwise you need to process the rows, and then:
<?php $stmt->nextRowset(); ?>

Once that is done for each returning rowset you will have access to the output parameters.

Please note, that PDO format numbers according to current locale. So if, locale set number format to something else, that standard that query WILL NOT work properly.

For example:
in Polish locale (pl_PL) proper decimal separator is coma (","), so: 123,45, not 123.45. If we try bind 123.45 to the query, we will end up with coma in the query.

<?php
setlocale(LC_ALL, 'pl_PL');
$sth = $dbh->prepare('SELECT name FROM products WHERE price < :price');
$sth->bindParam(':price', 123.45, PDO::PARAM_STR);
$sth->execute();
// result:
// SELECT name FROM products WHERE price < '123,45';
?>

Note that with bindParam the second parameter is passed by reference. This means that the following will produce a warning if E_STRICT is enabled:

<?php
$stmt->bindParam('type', $object->getType());

// Strict Standards: Only variables should be passed by reference in /path/to/file.php on line 123
?>

If the second parameter is not an actual variable, either set the result of $object->getType(); to a variable and use that variable in bindParam or use bindValue instead.

Do not try to use the same named parameter twice in a single SQL statement, for example

<?php
$sql = 'SELECT * FROM some_table WHERE  some_value > :value OR some_value < :value';
$stmt = $dbh->prepare($sql);
$stmt->execute( array( ':value' => 3 ) );
?>

...this will return no rows and no error -- you must use each parameter once and only once. Apparently this is expected behavior (according to this bug report: http://bugs.php.net/bug.php?id=33886)  because of portability issues.

A caution for those using bindParam() on a placeholder in a 
LIKE '%...%' clause, the following code will likely not work:

<?php
$q = "SELECT id, name FROM test WHERE name like '%:foo%'";
$s = "carrot";
$sth = $dbh->prepare($q);
$sth->bindParam(':foo', $s);
$sth->execute();
?>

What is needed is something like the following:

<?php
$s = "%$s%";
$sth->bindParam(':foo', $s);
?>

This should work. Tested against mysql 4.1, PHP 5.1.3.

Took me forever to find this elsewhere in the notes in the manual, so I'd thought I'd put this tidbit here to help others in the future.

When using a LIKE search in MySQL along with a prepared statement, the *value* must have the appropriate parentheses attached before the bindParam() statement as such:

<?php
$dbc = $GLOBALS['dbc'];
$sql = "SELECT * FROM `tbl_name` WHERE tbl_col LIKE ?";
$stmt = $dbc->prepare($sql);

$value = "%{$value}%";
$stmt->bindParam($i, $value, PDO::PARAM_STR);
?>

Trying to use
<?php
$stmt->bindParam($i, "%{$value}%", PDO::PARAM_STR);
?>

will fail.

/*
   method for pdo class connection, you can add your cases by    yourself and use it.
*/
class Conn{
....
....
private $stmt;
public function bind($parameter, $value, $var_type = null){
        if (is_null($var_type)) {
            switch (true) {
                               case is_bool($value):
                    $var_type = PDO::PARAM_BOOL;
                    break;
                case is_int($value):
                    $var_type = PDO::PARAM_INT;
                    break;
                case is_null($value):
                    $var_type = PDO::PARAM_NULL;
                    break;
                default:
                    $var_type = PDO::PARAM_STR;
            }
        }
        $this->stmt->bindValue($parameter, $value, $var_type);
    }

if you are storing files (or binary data), using PARAM_LOB (and moreover trying to do this with Oracle), don't miss this page :

http://php.net/manual/en/pdo.lobs.php

You will there notice that PDO-PGSQL and PDO-OCI don't work the same at all : not the same argument nor the same behaviour.

The documentation says this about the length parameter for bindParam:  

"To indicate that a parameter is an OUT parameter from a stored procedure, you must explicitly set the length. "

For db2, I found that setting the length for the "INPUT_OUTPUT" parameters causes a problem for varchar parameters that are input parameters.  The problem I found is that the stored procedure was called, but varchar input parameters were set to null inside my stored procedure and as a result, the stored procedure could not work properly.  

Here is the signature for my stored procedure:

CREATE OR REPLACE PROCEDURE MY_SCHEMA_NAME.MY_STORED_PROCEDURE_NAME ( IN RUN_ID INTEGER,IN V_SCHEMA_NAME VARCHAR(128),
  OUT out_rc INTEGER,OUT out_err_message VARCHAR(100),OUT out_sqlstate CHAR(5) ,OUT out_sqlcode INT)

Here is the php code that works:

$command = "Call MY_SCHEMA_NAME.MY_STORED_PROCEDURE_NAME (?,?,?,?,?,?,?)";
$stmt = $this->GuestDb->prepare($command);
$stmt->bindParam(1, $RUN_ID, PDO::PARAM_INT); 
$stmt->bindParam(2, $V_SCHEMA_NAME, PDO::PARAM_STR);
$stmt->bindParam(3, $V_TABNAME, PDO::PARAM_STR);
$stmt->bindParam(4, $out_rc, PDO::PARAM_INT|PDO::PARAM_INPUT_OUTPUT);
$stmt->bindParam(5, $out_err_message, PDO::PARAM_STR|PDO::PARAM_INPUT_OUTPUT);
$stmt->bindParam(6, $out_sqlstate, PDO::PARAM_STR|PDO::PARAM_INPUT_OUTPUT);
$stmt->bindParam(7, $out_sqlcode, PDO::PARAM_INT|PDO::PARAM_INPUT_OUTPUT);

Here is the php code that does not work:

$command = "Call MY_SCHEMA_NAME.MY_STORED_PROCEDURE_NAME (?,?,?,?,?,?,?)";
$stmt = $this->GuestDb->prepare($command);
$stmt->bindParam(1, $RUN_ID, PDO::PARAM_INT,12); 
$stmt->bindParam(2, $V_SCHEMA_NAME, PDO::PARAM_STR,128);
$stmt->bindParam(3, $V_TABNAME, PDO::PARAM_STR,100);
$stmt->bindParam(4, $out_rc, PDO::PARAM_INT|PDO::PARAM_INPUT_OUTPUT,12);
$stmt->bindParam(5, $out_err_message, PDO::PARAM_STR|PDO::PARAM_INPUT_OUTPUT,100);
$stmt->bindParam(6, $out_sqlstate, PDO::PARAM_STR|PDO::PARAM_INPUT_OUTPUT,6);
$stmt->bindParam(7, $out_sqlcode, PDO::PARAM_INT|PDO::PARAM_INPUT_OUTPUT,12);

MySQL will return an error if a named placeholder has a hyphen in it:
UPDATE wardrobe SET `T-Shirt`=:T-SHIRT WHERE id=:id

Will return the following error: PDOException' with message 'SQLSTATE[HY093]: Invalid parameter number: parameter was not defined'

To resolve, just remove hyphens from named placeholders:
UPDATE wardrobe SET `T-Shirt`=:TSHIRT WHERE id=:id

<?php
/**
 * Bind bit value:
 */

$sql = 'SELECT * FROM myTable WHERE level & ?';
$sth = \App::pdo()->prepare($sql);
$sth->bindValue(1, 0b0101, \PDO::PARAM_INT);
$sth->execute();
$result = $sth->fetchAll(\PDO::FETCH_ASSOC);

Spent all day banging my head against a brick wall.

Tried to use INOUT or OUT and getting the return variable into PHP using Mysql v5.5.16 on XAMPP.

"MySQL doesn't supporting binding output parameters via its C API.  You must use SQL level variables:"

<?php
$stm = $db->prepare("CALL sp_mysp(:Name, :Email, @sp_result)");

$outputArray = $db->query("select @sp_result")->fetch(PDO::FETCH_ASSOC);
?>

So the 'workaround' for Mysql and PDO is to use two SQL calls.

Hope this helps someone.

For those who are confused on insert query using PDO-bindparam:

$sql = $db->prepare("INSERT INTO db_fruit (id, type, colour) VALUES (? ,? ,?)");

$sql->bindParam(1, $newId);
$sql->bindParam(2, $name);
$sql->bindParam(3, $colour);
$sql->execute();

Examples doesn't highlight that this works as expected:

<?php
/* Execute a prepared statement by binding PHP variables */
$sth = $dbh->prepare('SELECT name, colour, calories
    FROM fruit
    WHERE calories < :calories AND colour = :colour');
$sth->bindParam('calories', $calories, PDO::PARAM_INT);
/* Names can be prefixed with colons ":" too (optional) */
$sth->bindParam(':colour', $colour, PDO::PARAM_STR);
$calories = 150;
$colour = 'red';
$sth->execute();
?>

calories get set to 150 in db, colour get set to red in db, because, as documentation points out: "the variable is bound as a reference and will only be evaluated at the time that PDOStatement::execute() is called."

🧨🧨🧨
<?php
$q = "SELECT id, name FROM test WHERE name like '%:foo%'";
$s = "carrot";
$sth = $dbh->prepare($q);
$sth->bindParam(':foo', $s);
$sth->execute();
?>

It's true, this ⬆ won't work but you can use concatenation ⬇:

<?php
$q = "SELECT id, name FROM test WHERE name like '%' || 
:foo || '%' ";
$s = "carrot";
$sth = $dbh->prepare($q);
$sth->bindParam(':foo', $s);
$sth->execute();
?>

If you're using the MySQL driver and have a stored procedure with an OUT or INOUT parameter, you can't (currently) use bindValue(). See http://bugs.php.net/bug.php?id=35935 for a workaround.

Here's how to build a dynamic WHERE LIKE at run time when the user can submit n keywords

<?php

if (array_key_exists('storyPieces', $_POST)) {
    $story_pieces = explode(',' $_POST['storyPieces']);
        foreach($story_pieces as $piece) {
        if ($conditional_count == 0) {
            $where_clause .= 'story LIKE ? ';
            $conditional_count++;
       } else {
            $where_clause .= 'AND story LIKE ? ';
            $conditional_count++;
       }
       }
}

// then after you've prepared

if (isset($story_pieces)) {
    foreach($story_pieces as $key => &$piece) {
        $piece = "%" . $piece . "%";
        $sth->bindParam($key + 1, $piece, PDO::PARAM_STR);
    }
}

?>