Very subtle PHP recursive calling code

<?php
Value that defines the angle of PI one point
Define ("PII", m_pi/180);

Creates a new image resource and defines its background as white, with a foreground color of black
$im = Imagecreate (670,500);
$white = Imagecolorallocate ($im, 0xFF, 0xFF, 0xFF);
$g = Imagecolorallocate ($im, 0x00, 0x00, 0x00);

The code instantiated from the following can be learned that the initial value $x, $y, $L, $a are divided into 300, 500, 100, 270
function Drawleaf ($g, $x, $y, $L, $a) {
Global $im;
$B = 50;
$C = 9;
$s 1 = 2;
$s 2 = 3;
$s 3 = 1.2;
if ($L > $s 1) {
Calculate the location of the leaves above
$x 2 = $x + $L * cos ($a * PII);
$y 2 = $y + $L * sin ($a * PII);
$x 2R = $x 2 + $L/$s 2 * cos (($a + $B) * PII);
$y 2R = $y 2 + $L/$s 2 * sin (($a + $B) * PII);
$x 2L = $x 2 + $L/$s 2 * cos (($a-$B) * PII);
$y 2L = $y 2 + $L/$s 2 * sin (($a-$B) * PII);

Calculate the location of the leaves below
$x 1 = $x + $L/$s 2 * cos ($a * PII);
$y 1 = $y + $L/$s 2 * sin ($a * PII);
$x 1L = $x 1 + $L/$s 2 * cos (($a-$B) * PII);
$y 1L = $y 1 + $L/$s 2 * sin (($a-$B) * PII);
$x 1R = $x 1 + $L/$s 2 * cos (($a + $B) * PII);
$y 1R = $y 1 + $L/$s 2 * sin (($a + $B) * PII);

Don't divide the trunk and foliage of the leaves.
Imageline ($im, (int) $x, (int) $y, (int) $x 2, (int) $y 2, $g);
Imageline ($im, (int) $x 2, (int) $y 2, (int) $x 2R, (int) $y 2R, $g);
Imageline ($im, (int) $x 2, (int) $y 2, (int) $x 2L, (int) $y 2L, $g);
Imageline ($im, (int) $x 1, (int) $y 1, (int) $x 1L, (int) $y 1L, $g);
Imageline ($im, (int) $x 1, (int) $y 1, (int) $x 1R, (int) $y 1R, $g);

Recursively call itself again
Drawleaf ($g, $x 2, $y 2, $L/$s 3, $a + $C);
Drawleaf ($g, $x 2R, $y 2R, $L/$s 2, $a + $B);
Drawleaf ($g, $x 2L, $y 2L, $L/$s 2, $a-$B);
Drawleaf ($g, $x 1L, $y 1L, $L/$s 2, $a-$B);
Drawleaf ($g, $x 1R, $y 1R, $L/$s 2, $a + $B);
}
}

Instantiation of
Drawleaf ($g, 300, 500, 100, 270);
Header ("Content-type:image/png");
Imagepng ($im);
?>

In my personal PHP programming experience, recursive calls are often used with static variables. The meaning of static variables can refer to the PHP manual. Hopefully, the following code will be more useful for understanding recursion and static variables.

Header ("Content-type:text/plain");
function Static_function () {
static $i = 0;
if ($i + + < 10) {
Echo $i. "\ n";
Static_function ();
}
}
Static_function (); This code outputs a number from 1 to 10. When the Static_function function is run for the second time, the variable i is still not released because it is a static variable, and thus the value of the increment can be obtained.