PHP - PDO problems with bindValue()

Question

I'm writing a class that programmatically creates a table given certain column names. I'm using PDO prepared statements which seem to be causing some problems.

Here's the basic procedure:

// create a query string to be sent to $pdo->prepare
$sql =
'CREATE TEMP TABLE :tmp_table_name (
  :person_id bigint,
  :encntr_id bigint,
  :prsnl_id bigint,
  :program_detail text,
  :program_name text
);';
$stmt = $pdo->prepare($sql);

// Bind table name
$stmt->bindValue(':tmp_table_name', 'tmp_patients', PDO::PARAM_STR);

// Bind column names
$columnNames = [
  'person_id',
  'encnt_id',
  'prsnl_id',
  'program_detail',
  'program_name',
];
foreach($columnNames as $name) {
    $stmt->bindValue(':'.$name, $name, PDO::PARAM_STR);
}

$ret = $stmt->execute();
// $ret is false! The statement fails.

Here's what $stmt->errorInfo()[2] displays:

ERROR:  syntax error at or near "$1"
LINE 1: CREATE TEMP TABLE $1 ($2 bigint,
$3 bigint,
$4 bigint,
$5 text,
$6 te...

Why are $1, $2, $3, etc. showing up in the query? Do you have any tips on how to further debug this PDO statement?

Update I'm trying a different approach, not using bindParam, just passing in an array of params to $stmt->execute(). I'm still getting the same error, though...

$stmt = $this->pdo->prepare($this->createSql);

$keys = [];
// Bind column names
foreach($this->columnNames as $name) {
    $keys[] = ':'.$name;
}
$params = array_combine($keys, $this->columnNames);

// Bind table name
$params[':tmp_table_name'] = 'tmp_'.$this->objName;

$ret = $stmt->execute($params);
if (!$ret) {
    throw new Exception('execSchema() failed! '. $stmt->errorInfo()[2]);
}

Answer 1

It looks to me as if you are trying to bind to the table name/columns, which you cannot do using PDO - you can only bind values to placeholders.

as seen in your code here:

// Bind table name
$stmt->bindValue(':tmp_table_name', 'tmp_patients', PDO::PARAM_STR);

It feels as though it should work, but it does not.

eg this will not work:

$sql = "insert into mytable (:thing1) values (:thing2);
$stmt->bindValue(':thing1', 'column_name' ); // this would not
$stmt->bindValue(':thing2', 'column_value' ); // this would have (on its own)

However, something like this would work, which should proffer your solution or something like it:

// imagine an incoming POST load like this:
$cols = ['fname','lname'];

$array_cnt = count($cols);  // 2
$sql = 'INSERT INTO mytable ('.join(',', $cols).') ';

// and then that number of placeholders used 
$sql .= 'VALUES (' . join( array_fill( 0, $array_cnt, '?' ), ', ' ) . ')'; 

echo $sql;

Answer 2

bindParam() and bindValue() work the value by-reference, which means you passed it a reference to the $value variable, Not its value at the time you called it. When the loop ends and you call query(), $value will be whatever the last one in the $array was.

Use a reference to the item in the array:

$stmt->bindValue(":".$array[$param], $array[$param]);
$stmt->bindParam($param, $array[$param]);