As people have mentioned in the comments, your code is filled with all sorts of errors. What I believe you want is this:
class Person {
   function __construct($firstname,$lastname,$age) {
       $this->isAlive   = true;
       $this->firstname = $firstname;
       $this->lastname  = $lastname;
       $this->age       = $age;
    }
}
$teacher = new Person("boring", "12345", 12345);
$student = new Person("boringw", "12345w", 12345);
print_r($teacher);
This code has been tested and it works.
Results:
Person Object ( [isAlive] => 1 [firstname] => boring [lastname] => 12345 [age] => 12345 )
Note: You cannot echo an object.
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solved php class error, it wont echo teacher [closed]