How do I use a function that I defined in a parent class in the child class?

for example if i use a class like the below

```
<?php

class mat

{

function square($x)

{

return $x *$x;

}

}

class matchild extends mat

{

function doublesquare($x)
{

return square($x) * square($x)

}

}

?>

```

If I try the above , I get an error saying that the square function is not defined. 

Answers and suggestions appreciated.