What Do Underscores in a PHP Number Mean?

Introduced in PHP 7.4, an underscore (_) can be used as a numeric separator to act as a visual separation between groups of digits. The purpose of having numeric separators is to improve readability of numeric literals. For example:

// PHP 7.4+
$million = 1_000_000;

echo $million; // 1000000
var_dump($million === 1000000); // true

Numeric separators can be used with the following numeric literals:

• Decimal;
• Float;
• Scientific Notation;
• Binary;
• Octal;
• Hexadecimal.

Numeric separators have no semantic meaning, beyond improving readability of (long) numeric literals. In fact, numeric literals are parsed as if the underscores were absent:

// PHP 7.4+
// decimal
echo 1_000_000; // 1000000

// float
echo 1_000.123_456; // 1000.123456

// scientific
echo 1.23_45e+6; // 1234500

// PHP 7.4+
// binary
echo 0b10_11; // 11
echo 0B10_11; // 11

// octal
echo 0_30_071; // 12345
echo 030_071; // 12345
echo 0o30_071; // 12345 (PHP 8.1+)
echo 0O30_071; // 12345 (PHP 8.1+)

// hexadecimal
echo 0xddd_5; // 56789
echo 0Xddd_5; // 56789

When using an underscore (_) between digits, the following are considered invalid:

• When there's more than one underscore between any two digits;
• When an underscore is not directly between two digits;
• When a numeric literal starts or ends with an underscore;
• When an underscore is used before or after a decimal point (or other "special positions" such as, exponent, number sign, etc.).

For example:

// PHP 7.4+
// Parse error: syntax error, unexpected identifier "__000"
100__000;

// PHP 7.4+
// Warning: Use of undefined constant _1234 - assumed '_1234' (this will throw an Error in a future version of PHP)
_1234;
+_1234;
-_1234;

// PHP 8+
// Fatal error: Uncaught Error: Undefined constant "_1234"
_1234;
+_1234;
-_1234;

// PHP 7.4+
// Parse error: syntax error, unexpected identifier "_"
100_;

// PHP 7.4+
// Parse error: syntax error, unexpected identifier "_"
1_.23;

// PHP 7.4+
// Parse error: syntax error, unexpected identifier "e_23"
1e_23;

// Parse error: syntax error, unexpected identifier "_e23"
1_e23;

// Parse error: syntax error, unexpected identifier "e"
1e+_23;
1e-_23;

// PHP 7.4+
// Parse error: syntax error, unexpected identifier "..."
0b_10_11;
0B_10_11;
0o_30_071;
0O_30_071;
0x_ddd_5;
0X_ddd_5;