Calling a function with dollar sign in front

I’m new to php, and I found a tutorial about cropping image, with a strange instruction I never see. I don’t know how to search about it.

$src_img = $image_create($source_file);

Here it is the full code of the tutorial

 //resize and crop image by center
function resize_crop_image($max_width, $max_height, $source_file, $dst_dir, $quality = 80){
    $imgsize = getimagesize($source_file);
    $width = $imgsize[0];
    $height = $imgsize[1];
    $mime = $imgsize['mime'];

    switch($mime){
        case 'image/gif':
            $image_create = "imagecreatefromgif";
            $image = "imagegif";
            break;

        case 'image/png':
            $image_create = "imagecreatefrompng";
            $image = "imagepng";
            $quality = 7;
            break;

        case 'image/jpeg':
            $image_create = "imagecreatefromjpeg";
            $image = "imagejpeg";
            $quality = 80;
            break;

        default:
            return false;
            break;
    }

    $dst_img = imagecreatetruecolor($max_width, $max_height);
    $src_img = $image_create($source_file);

    $width_new = $height * $max_width / $max_height;
    $height_new = $width * $max_height / $max_width;
    //if the new width is greater than the actual width of the image, then the height is too large and the rest cut off, or vice versa
    if($width_new > $width){
        //cut point by height
        $h_point = (($height - $height_new) / 2);
        //copy image
        imagecopyresampled($dst_img, $src_img, 0, 0, 0, $h_point, $max_width, $max_height, $width, $height_new);
    }else{
        //cut point by width
        $w_point = (($width - $width_new) / 2);
        imagecopyresampled($dst_img, $src_img, 0, 0, $w_point, 0, $max_width, $max_height, $width_new, $height);
    }

    $image($dst_img, $dst_dir, $quality);

    if($dst_img)imagedestroy($dst_img);
    if($src_img)imagedestroy($src_img);
}
//usage example
resize_crop_image(100, 100, "test.jpg", "test.jpg");

Answer

$image_create returns a string.

This sting is a dynamic function (whose name is decided run time)

Reference:

http://php.net/manual/en/functions.variable-functions.php

Instead of adding 3 if statements for choosing three functions: imagecreatefromgif(), imagecreatefrompng() and imagecreatefromjpeg(), a variable is taken which will switch the function variable (name) and that will be used.

Which is easier to use.