E

The letter e, or E, plays several distinct roles:

  • Floating number format: e is used as part of a float literal to denote a power of 10. 1.5e3 means 1.5 × 10³ = 1500, and 7E-10 means 7 × 10⁻¹⁰. The letter is case-insensitive in this context. It is not an operator; it is part of the numeric syntax

  • Exponential constant: M_E is a PHP predefined math constant equal to Euler’s number e ≈ 2.718281828459045. It is the base of the natural logarithm, and satisfies log(M_E) === 1. The function exp(1) also returns this value, although floating-point rounding means exp(1) === M_E may evaluate to false

  • Euler constant: M_EULER is a distinct PHP constant, equal to the Euler constant γ ≈ 0.5772156649015329. It is also called Euler-Mascheroni, or gamma. It should not be confused with M_E

  • Error constant prefix: all PHP error level constants start with E_: E_ERROR, E_WARNING, E_NOTICE, E_DEPRECATED, E_ALL, and others. They are used with error_reporting() and combined with bitwise operators

  • Escape sequence for escape character: in double-quoted strings and heredocs, \e is the escape sequence for the ESC character, ASCII 27, 0x1B. It is used in ANSI terminal escape codes.

<?php

    // e in float scientific notation
    $a = 1.5e3;   // 1500.0
    $b = 7E-10;   // 0.0000000007
    $c = 1e308;   // close to PHP_FLOAT_MAX

    // M_E : Euler's number
    echo M_E;             // 2.718281828459
    echo exp(1);          // 2.718281828459
    echo log(M_E);        // 1

    // M_EULER : Euler-Mascheroni constant (different from M_E)
    echo M_EULER;         // 0.57721566490153

    // E_ error level constants
    error_reporting(E_ALL);
    error_reporting(E_ERROR | E_WARNING);

    // \e escape sequence : ESC character for ANSI terminal codes
    echo "\e[31mRed text\e[0m\n";

?>

Documentation

See also Math constants and Float syntax.

Related : Floating Point Numbers, Exponential, Logarithm, Not A Number (NAN), INF, Error Reporting, Escape Sequences, Exponent