为什么我收到Undefined Variable错误？

Question

I know that it's not necessary to define variables and arrays before using them in PHP. But I'm facing 'Undefined Variable mat1' and 'Undefined Variable mat2' errors when I run the following code:

for($i=0;$i<3;$i++)
{
   for($j=0;$j<3;$j++)
   {
       $ans[$i][$j] = 0;
       for($k=0;$k<3;$k++) 
       {
          $ans[$i][$j] = $ans[$i][$j] + $mat1[$i][$k]*$mat2[$k][$j];
       }
   }
}
var_dump($ans);

I tried to define them with following 2 lines:

$mat1=array(array());
$mat2=array(array());

But errors were changed to 'Undefined offset: ...' errors. Am I missing something in my code?!

Answer 1

Why I'm getting Undefined Variable errors?

Because you haven't defined anything.

Just use

$mat1 = $mat2 = array(
            0 => array(
                    0 => 1, 
                    1 => 1, 
                    2 => 1, 
                    3 => 1, 
                    4 => 1, 
                    5 => 1, 
                ),
            1 => array(
                    0 => 2, 
                    1 => 2, 
                    2 => 2, 
                    3 => 2, 
                    4 => 2, 
                    5 => 2, 
                    ),
            2 => array(
                    0 => 3, 
                    1 => 3, 
                    2 => 3, 
                    3 => 3, 
                    4 => 3, 
                    5 => 3, 
            )
    );

And test it