if(!isset($string1)) checks if the variable is set and not NULL
unset($string3) Destroys the specified variable.
(integer)$num rounded to integer
settype($num,'integer') use the settype function to convert the type
is_numeric checks if a variable is a number or a string of numbers
$i = 'only seen once'; echo "$i"."<p>"; echo "$i"; output only once
Want to enter the $ number: echo "Yes, the \$boo is a phone number: $boo!";
define ("MESSAGE","I am a PHP programmer",boolean) Define a constant,boolean (set case insensitive)
echo __FILE__ (file path) __LINE__ (current line number) PHP_VERSION (PHP version) PHP_OS (current system)
$j = & $i; Reference assignment, after the following, as long as i changes, j will also change with it
Locally declared variables do not affect global variables
static $message = 0; set static, $message+=1; the for loop can accumulate (Chapter 3, Lesson 14)
When a local variable wants to call a global variable, add global $hr;
$a = "b"; $b = "I like PHP"; echo $$a."<p>"; output the value of $b through a variable
$a = -100; $c = 30; echo ($a % $c) outputs -10, divide 100 by 3, use 3 to get 90, leaving 10;
$n = "3.1415926r*r"; $m = 1; $nm = $n.$m; $mn = $n + $m ;