if statement with just one variable as conditionals-php -

i trying study php , came across if statement has single variable conditional this:

<?php if($a){ //do } ?>; 

i know c++ , javascript, in languages, if statement ben invalid. tell me if statement means?

as rizier123 said above, conditional checking "truthy" values in variable $a. in loosely-typed language php, truthy value non-empty, non-null, or non-zero value.

see great answer similar question:
https://stackoverflow.com/a/6693908/2796449
or php docs:
http://php.net/manual/en/language.types.boolean.php

boolean logic reduces down entire statement down 1 value, either true or false. executing "echo (3==4);" show false. if variable returns truthy value, evaluated true.

these 3 comparable statements:

$a = 1;
if ( $a )

if ( 1 )

if ( true )