How to create strong blur using the image convolution function

Question

How to create strong blur using the image convolution function

I am trying to create a really blurry image using the convolution of images of a PHP function. This image is used as the background behind the image in the slider.

I tried using the Image Filter, but the Gaussian Blur and Selective Blur are very light and you cannot change the blur.

This is the code that I am currently using to create blur, but it is not really blurry.

    $matrix = array( array(5, 5, 5), array(0, 0, 0), array(5, 5, 5));
    for($i = 1; $i<=20; $i++) {
        imageconvolution($image, $matrix, 30, 0);
     }