A variable is just a storage element.

Variables in PHP are represented by a dollar sign ( $ ) by the name of the variable. The variable name is case-sensitive.

The variable name should be start with letters ( a-zA-Z ) or underscore ( _ ) and it should not start with numeric /number

Below we have given some examples for your reference.

<?php
$x = 'hello';
$$x = 'world';
echo "$x ${$x}";
echo "$x $hello";
?>

Output:
hello world
hello world

<?php
class test {
    var $bar = 'I am bar.';
    var $arr = array('I am A.', 'I am B.', 'I am C.');
    var $r   = 'I am r.';
}

$test = new test();
$bar = 'bar';
$baz = array('foo', 'bar', 'baz', 'quux');
echo $test->$bar . "\n";
echo $test->$baz[1] . "\n";

$start = 'b';
$end   = 'ar';
echo $test->{$start . $end} . "\n";

$arr = 'arr';
echo $test->$arr[1] . "\n";
echo $test->{$arr}[1] . "\n";

?>

Output:
I am bar.
I am bar.
I am bar.
I am r.
I am B.

<?php
$a = 10;
$b = 15;
echo $c = $a + $b;

function myTest()
{
global $a,$a;
$d=$a+$b;
}

myTest();
echo $d;

function myTest()
{
$GLOBALS['b']=$GLOBALS['a']+$GLOBALS['b'];
} 

myTest();
echo $b;

?>
Output:
25
25
25