Integers

An 
int is a number of the set ℤ = {..., -2, -1, 0, 1, 2, ...}.
ENTITY
Floating-point numbers, Arbitrary precision / BCMath, Arbitrary length integer / GMP
Syntax
Ints can be specified in decimal (base 10), hexadecimal (base 16), octal (base 8) or binary (base 2) notation. The negation operator can be used to denote a negative 
int.
To use octal notation, precede the number with a 
0 (zero). As of PHP 8.1.0, octal notation can also be preceded with 
0o or 
0O. To use hexadecimal notation, precede the number with 
0x. To use binary notation, precede the number with 
0b.
As of PHP 7.4.0, integer literals may contain underscores (
_) between digits, for better readability of literals. These underscores are removed by PHP's scanner.
Integer literals
<?php
$a = 1234; // decimal number
$a = 0123; // octal number (equivalent to 83 decimal)
$a = 0o123; // octal number (as of PHP 8.1.0)
$a = 0x1A; // hexadecimal number (equivalent to 26 decimal)
$a = 0b11111111; // binary number (equivalent to 255 decimal)
$a = 1_234_567; // decimal number (as of PHP 7.4.0)
?>
   
Formally, the structure for 
int literals is as of PHP 8.1.0 (previously, the 
0o or 
0O octal prefixes were not allowed, and prior to PHP 7.4.0 the underscores were not allowed):
decimal     : [1-9][0-9]*(_[0-9]+)*
            | 0
hexadecimal : 0[xX][0-9a-fA-F]+(_[0-9a-fA-F]+)*
octal       : 0[oO]?[0-7]+(_[0-7]+)*
binary      : 0[bB][01]+(_[01]+)*
integer     : decimal
            | hexadecimal
            | octal
            | binary
The size of an 
int is platform-dependent, although a maximum value of about two billion is the usual value (that's 32 bits signed). 64-bit platforms usually have a maximum value of about 9E18. PHP does not support unsigned 
ints. 
int size can be determined using the constant 
PHP_INT_SIZE, maximum value using the constant 
PHP_INT_MAX, and minimum value using the constant 
PHP_INT_MIN.
Integer overflow
If PHP encounters a number beyond the bounds of the 
int type, it will be interpreted as a 
float instead. Also, an operation which results in a number beyond the bounds of the 
int type will return a 
float instead.
Integer overflow
<?php
$large_number = 50000000000000000000;
var_dump($large_number);         // float(5.0E+19)
var_dump(PHP_INT_MAX + 1);       // 32-bit system: float(2147483648)
                                 // 64-bit system: float(9.2233720368548E+18)
?>
   
Integer division
There is no 
int division operator in PHP. To achieve this, use the 
intdiv function. 
1/2 yields the 
float 
0.5. The value can be cast to an 
int to round it towards zero, or the 
round function provides finer control over rounding.
Divisions
<?php
var_dump(25/7);         // float(3.5714285714286)
var_dump((int) (25/7)); // int(3)
var_dump(round(25/7));  // float(4)
?>
   
Converting to integer
To explicitly convert a value to 
int, use the 
(int) cast. However, in most cases the cast is not needed, since a value will be automatically converted if an operator, function or control structure requires an 
int argument. A value can also be converted to 
int with the 
intval function.
If a 
resource is converted to an 
int, then the result will be the unique resource number assigned to the 
resource by PHP at runtime.
See also Type Juggling.
From booleans
ENTITY will yield 
0 (zero), and ENTITY will yield 
1 (one).
From floating point numbers
When converting from 
float to 
int, the number will be rounded towards zero. As of PHP 8.1.0, a deprecation notice is emitted when implicitly converting a non-integral ENTITY to an ENTITY which loses precision.
Casting from Float
<?php
function foo($value): int {
  return $value;
}
var_dump(foo(8.1)); // "Deprecated: Implicit conversion from float 8.1 to int loses precision" as of PHP 8.1.0
var_dump(foo(8.1)); // 8 prior to PHP 8.1.0
var_dump(foo(8.0)); // 8 in both cases
var_dump((int) 8.1); // 8 in both cases
var_dump(intval(8.1)); // 8 in both cases
?>
    
If the float is beyond the boundaries of 
int (usually 
+/- 2.15e+9 = 2^31 on 32-bit platforms and 
+/- 9.22e+18 = 2^63 on 64-bit platforms), the result is undefined, since the 
float doesn't have enough precision to give an exact 
int result. As of PHP 8.5.0, an 
E_WARNING is emitted when a 
float that is not representable as an 
int is cast to one. Prior to PHP 8.5.0, no diagnostic at all was issued.
[!NOTE]
NaN, 
Inf and 
-Inf will always be zero when cast to 
int. As of PHP 8.5.0, casting them to 
int emits an 
E_WARNING, as does casting 
NAN to any other type.
[!WARNING]
Never cast an unknown fraction to 
int, as this can sometimes lead to unexpected results.
<?php
echo (int) ( (0.1+0.7) * 10 ); // echoes 7!
?>
     
See also the warning about float precision.
From strings
If the string is numeric or leading numeric then it will resolve to the corresponding integer value, otherwise it is converted to zero (
0).
From 
NULL
ENTITY is always converted to zero (
0).
From other types
[!CAUTION]
The behaviour of converting to 
int is undefined for other types. Do not rely on any observed behaviour, as it can change without notice.