Single¶

Single usually means that something appears only once.

Single is opposed to double, but it may also be distinct with any other number of repetition, such as triple, quadruple, etc.

Things that may be single:

Single quotes \'
Single equal =, not to be confused with the double equal == or the triple one ===
Single colon :, to identify goto labels or ternary operators
Single plus + and minus -, for addition and substraction
Single star *, for multiplication
Single question mark ??, for ternary operators.

<?php

    $a = 1;
    $a == 1;  // true
    $a === 1; // true too.

?>