<?php
interface abc {
   public function setVariable($name, $var);
   public function getVariable($name);
}

class my_abc implements abc {
   private $vars = array();

   public function setVariable($name, $var) {
       $this->vars[$name] = $var;
   }

   public function getVariable($name) {
       return $this->vars[$name];
   }
}
$test = new my_abc;
$test->setVariable('foo','bar');
echo $test->getVariable('foo');
?>
Output
bar
Now, how about if we don't implement the interface correctly:

<?php
class my_other_class implements abc {
    function __construct() {
        $this->test = 'something';
    }
}
?>
Fatal error: Class my_other_class contains 2 abstract methods and must therefore be declared abstract (abc::setVariable, abc::getVariable) in script.php on line 2