call_user_func_array passing arguments to a constructor [duplicate]

Question

I have searched many a page of Google results as well as here on stackoverflow but cannot find a solution that seems to fit my situation. I appear to have but one last snag in the function I am trying to build, which uses call_user_func_array to dynamically create objects.

The catchable fatal error I am getting is Object of class Product could not be converted to string. When the error occurs, in the log I get five of these (one for each argument): PHP Warning: Missing argument 1 for Product::__construct(), before the catchable fatal error.

This is the code of the function:

public static function SelectAll($class, $table, $sort_field, $sort_order = "ASC")
{  
/* First, the function performs a MySQL query using the provided arguments. */

$query = "SELECT * FROM " .$table. " ORDER BY " .$sort_field. " " .$sort_order;
$result = mysql_query($query);

/* Next, the function dynamically gathers the appropriate number and names of properties. */

$num_fields = mysql_num_fields($result);
for($i=0; $i < ($num_fields); $i++)
{
  $fetch = mysql_fetch_field($result, $i);
  $properties[$i] = $fetch->name;
}

/* Finally, the function produces and returns an array of constructed objects.*/

while($row = mysql_fetch_assoc($result))
{
  for($i=0; $i < ($num_fields); $i++)
  {
    $args[$i] = $row[$properties[$i]];
  }
  $array[] = call_user_func_array (new $class, $args);
}

return $array;
}

Now, if I comment out the call_user_func_array line and replace it with this:

$array[] = new $class($args[0],$args[1],$args[2],$args[3],$args[4]);

The page loads as it should, and populates the table I am building. So everything is absolutely functional until I try to actually use my $args array within call_user_func_array.

Is there some subtle detail about calling that array that I am missing? I read the PHP manual for call_user_func_array once, and then some, and examples on that page seemed to show people just building an array and calling it for the second argument. What could I be doing wrong?

Answer 1

You can't call the constructor of $class like this:

call_user_func_array (new $class, $args);

That's no valid callback as first parameter. Let's pick this apart:

call_user_func_array (new $class, $args);

Is the same as

$obj = new $class;
call_user_func_array ($obj, $args);

As you can see, the constructor of $class has been already called before call_user_func_array comes into action. As it has no parameters, you see this error message:

Missing argument 1 for Product::__construct()

Next to that, $obj is of type object. A valid callback must be either a string or an array (or exceptionally a very special object: Closure, but that's out of discussion here, I only name it for completeness).

As $obj is an object and not a valid callback, so you see the PHP error message:

Object of class Product could not be converted to string.

PHP tries to convert the object to string, which it does not allow.

So as you can see, you can't easily create a callback for a constructor, as the object yet not exists. Perhaps that's why you were not able to look it up in the manual easily.

Constructors need some special dealing here: If you need to pass variable arguments to a class constructor of a not-yet initialize object, you can use the ReflectionClass to do this:

  $ref = new ReflectionClass($class);
  $new = $ref->newInstanceArgs($args);

See ReflectionClass::newInstanceArgs

Answer 2

Not possible using call_user_func_array(), because (as the name suggest) it calls functions/methods, but is not intended to create objects, Use ReflectionClass

$refClass = new ReflectionClass($class);
$object = $refClass->newInstanceArgs($args);

Another (more design-based) solution is a static factory method

class MyClass () {
  public static function create ($args) {
    return new self($args[0],$args[1],$args[2],$args[3],$args[4]);
  }
}

and then just

$object = $class::create($args);

In my eyes it's cleaner, because less magic and more control