I'll tell you some things.

A. Regarding the error you have in question

In PHP the functions can receive arguments (simple variables, fixes, etc):

  

Function arguments

     

Any information can be passed to the functions through the   list of arguments, which is a list of delimited expressions   by commas. The arguments are evaluated from left to right.

In your case, the function needs to know the argument with which it will work. If it is not declared within the function and it is not a variable of the global type, then you have to pass it to it when you call the function:

Function:

function mostrarMatriz($m)
{
   for ($i = 0; $i < $m->getFilas(); $i++)
   {
      //...
   }
}

Calling the function:

$m = new Matriz();
mostrarMatriz($m);

With this your problem would be solved.

B. But ...

It is possible to do it in another way. Seen the code, one wonders why not create the function mostrarMatriz within Clase Matriz() ?

It would not be a bad practice to create a class inside a getter of the complete matrix, in fact, this is what the OOP recommends (when we work with classes). In this way your Matrix class would have different ways of obtaining the data. Just as you have a method getFilas() , getColumnas() , you could have a method getMatriz() that would return the matrix.

For this the function should have a return.

EDIT:

Possible Model of a Parent Class

class Matriz 
{
    private $filas;
    private $columnas;

    /* 
        *Constructor con parámetros los cuales deben ser pasados 
        * cuando se crea una instancia de la clase
        * usando por ejemplo: $m = new Matriz ("10", "60");
        * nótese que el tipo de dato puede ser otro que String, indicándolo 
        * debidamente a la clase
    */

    public function __construct($filas, $columnas) 
    {
        $this->filas = $filas;
        $this->columnas = $columnas;
        //... todas las demás variables que formarán parte de la clase y que se reciben en parámetro
        // ... las demás, ejemplo las que devuelven los métodos públicos no necesitan declararse
        // ... aquí sólo las variables que la clase necesita *conocer* para trabajar con ellas
    }

    //En este caso el método no necesita parámetros
    public function getMatriz() 
    {

        for ($i = 0; $i < $this->filas; $i++)
        {
        //... aquí se crea una representación de la matriz en una variable o lo que se quiera
        // es lo que se enviará en el return
        }
        return $variableconcatenadaenelbuclefor
    }


    public function getFilas() 
    {
        return $this->filas;

    }


    public function getColumnas() 
    {
        return $this->columnas;

    }
}

As you see, in the class the other methods that the constructor do not receive any parameter, and we can access the rows / columns received in the constructor using the pseudo variable $this .

Example of use:

//Se pasan los valores al contructor, en este caso los valores filas y columnas
$m = new Matriz ("10", "50");
echo "Filas: ". $m->getFilas();
echo "Columnas: ". $m->getColumnas();
echo "Matriz: ". $m->getMatriz();

That happens to you because the variable $m defines it outside the function. A function "isolates" the variables, so the block for can not "see" a $m .

The solution is to pass $m as a parameter of the function mostrarMatriz() , like this:

function mostrarMatriz ($m) {  

The rest of the code would remain the same.