What's different between "isset($string)" & "@$string" on PHP?

Question

What's different between isset($string) and @$string in PHP ?

This works both ways, so what's the difference?

if(isset($string))
 ....


if(@$string)
 ....

Answer 1

1. isset is true if the given variable exists and is not null.
   @$var is true if the value of $var is true in a loose comparison.*
   !== null and == true matches different values.

2. In the case of @$var, if $var does not exist, an error will be generated internally and raised and its output suppressed through @. This is a much more expensive operation, and it may trigger custom defined error handlers on the way. Do not ever use it if there's an alternative.

* A non-existent variable's value is substituted by null, which equals false.

Answer 2

Using isset($str) is not the same as @$str.

isset($str) is true if $str is set and is not null.
@$str is true if $str is truthy.

Consider the following example:

$str = "0";

if (isset($str)) {
    // This gets printed because $str is set
    echo "Str is set" . PHP_EOL;
}

if (@$str) {
    // This is NOT printed because $str is falsy
    echo "Str is truthy" . PHP_EOL;
}

It should also be noted that @, in addition to being a bad habit, incurs a significant performance penalty.