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Variables and Constants

All variable names are preceded by the $ symbol. Variables need not be declared before values can be assigned to them:
Variable assignment examples
$a = trUE; // boolean
$b = Null;  // NULL
$c = 033;  // octal integer
$d = -0x1C;  // hexadecimal integer
$e = 0b101; // binary integer
$f = 1.331;  // float
$g = 1234e-3;  // exponential float
$h = intval(“5.7”); # 5, integer conversion
$i = isset($a,$b,$c); # false, $b is NULL
$j = empty($c);  #false, $c is not false
$k = empty($z); # true, $z is undefined
                            
While the names of variables are case-sensitive, the values true, false and NULL are not. 0, the empty string””, the string “0”, an array with zero elements and NULL are considered false.

The size of an integer can be determined using PHP_INT_SIZE. The maximum allowed value of an integer can be determined using PHP_INT_MAX. If an integer overflows, it will be interpreted as a float instead.

Use &$variable for assignment by reference. For example, {$a=4; $b= &$a; $a=5;} will result in $b getting a value of 5 since $a and $b now point to the same value. The bindings of variables may be broken with the unset() function, eg.: unset($a,$b). Note that unsetting $a alone will not unset $b after the assignment by reference.

Constants are identifiers with values that do not change. The names of constants do not start with $. Their values can be directly accessed anywhere, even in a function:
Constant definition examples
define ("a",100); // one way to define a constant
const b=3; // another way to define a constant 
echo a*b; // one way to use a constant
echo constant(“b”); //another way to use it
echo defined(“a”); // true


Since PHP 7.4, an underscore (_) can be used inside numeric literals as a visual separator between digits, making long numbers easier to read. It has no effect on the value, and is allowed in integers, floats, and in binary/octal/hexadecimal literals – as long as it sits between two digits (never leading, trailing, or doubled up). 

<?php
$million = 1_000_000;
$price = 1_234.567_8;
$binary = 0b0100_1010;
$hex = 0xCAFE_F00D;

echo $million . "\n";
echo $price . "\n";
echo $binary . "\n";
echo $hex . "\n";
?>

1000000 1234.5678 74 3405705229
PHP has always allowed octal integers via a leading 0, e.g. 016. Since PHP 8.1, an explicit octal notation using a 0o (or 0O) prefix is also available, matching the existing 0x (hexadecimal) and 0b (binary) prefixes and making the base obvious at a glance. Both forms produce the identical integer value. 

<?php
$oldStyle = 016;
$newStyle = 0o16;

echo $oldStyle . "\n";
echo $newStyle . "\n";
var_dump($oldStyle === $newStyle);
?>

14 14 bool(true)