7.12. Creating a Class Dynamically
Problem
You want to create a class, but you don’t know everything about it until your code is executed.
Solution
Use eval( )
with interpolated variables:
eval("class van extends $parent_class { function van() { \$this->$parent_class(); } };"); $mystery_machine = new van;
Discussion
While it’s okay in PHP to use variable names to call functions or create objects, it’s not okay to define functions and classes in a similar manner:
$van( ); // okay $van = new $parent_class // okay function $van( ) {}; // bad class $parent_class {}; // bad
Trying to do either of the last two examples results in a parser error because PHP expects a string, and you supplied a variable.
So, if you want to make a class named $van
and you
don’t know beforehand what’s going
to be stored in $van
, you need to employ
eval( )
to do your dirty work:
eval("class $van {};");
There is a performance hit whenever you call eval( )
, so high traffic sites should try to restructure their
code to avoid this technique when possible. Also, if
you’re defining your class based on input from
users, be sure to escape any potentially dangerous characters.
See Also
Recipe 7.14 to instantiate an object
dynamically; documentation on eval( )
at
http://www.php.net/eval.
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