Use var_dump() to assist in checking imageopenpolygon() input parameters

During PHP graphics processing, imageopenpolygon() is a function used to draw open polygons (Open Polygon). But sometimes, when passing parameters, the graph cannot be drawn correctly, or the code runs incorrectly. In order to quickly locate the problem, we can use var_dump() to assist in checking whether the input parameters of imageopenpolygon() are correct.

Understand the parameters of imageopenpolygon()

First, let’s take a look at the basic definition of imageopenpolygon() :

 bool imageopenpolygon(
    GdImage $image,
    array $points,
    int $num_points,
    int $color
)

• $image is an image resource created by imagecreate() or imagecreatetruecolor() .

• $points is an array of points in the format [x0, y0, x1, y1, ..., xn, yn] .

• $num_points is the number of points (not the number of array elements, but the number of coordinate pairs).

• $color is the color identifier returned by imagecolorallocate() .

Common errors include:

• $points array length is not even.

• $num_points calculation error.

• The value in $points is not an integer.

How to use var_dump() to assist with checking?

var_dump() can clearly display the types and values ​​of variables, making it ideal for debugging data of complex arrays or resource types.
Before calling imageopenpolygon() , you can check the passed parameters like this:

Sample code:

 <?php
// Create a blank image
$image = imagecreatetruecolor(200, 200);

// Assign colors
$color = imagecolorallocate($image, 255, 0, 0);

// Define a point array
$points = [
    50,  50,
    150, 50,
    150, 150,
    50,  150
];
$num_points = count($points) / 2;

// use var_dump Check parameters
var_dump($points);
var_dump($num_points);
var_dump($color);

// Try drawing open polygons
if (!imageopenpolygon($image, $points, $num_points, $color)) {
    echo "Drawing failed，Please check the input parameters。";
} else {
    // Output pictures to browser
    header('Content-Type: image/png');
    imagepng($image);
}

// Free memory
imagedestroy($image);
?>

After running the output example

 array(8) {
  [0]=>
  int(50)
  [1]=>
  int(50)
  [2]=>
  int(150)
  [3]=>
  int(50)
  [4]=>
  int(150)
  [5]=>
  int(150)
  [6]=>
  int(50)
  [7]=>
  int(150)
}
float(4)
int(16711680)

With such output, you can quickly confirm:

• Whether $points is a numerical pair and is arranged correctly.

• Whether $num_points is the correct integer (note that if the calculation is a floating point number, it needs to be cast to an integer).

• Whether $color has been assigned correctly.

If the $points array data is incorrect, such as an odd number of elements, or the coordinate data is a string (such as "50px" ), you can immediately discover and correct it.

Things to note

1. The num_points parameter should be an integer. If the decimal obtained by division is used, remember to force transformation with (int) .

2. Confirm that the total number of elements in the $points array is even.

3. Confirm that the color resource $color is correctly allocated.

4. Do not output var_dump() in a formal environment, you can use logs instead, for example: