Why Variable value is not changed after calling the function

Question

If a variable is defined outside a function and again it defined inside the function then why the value of variable is not changed after calling the function.

$a = 12;

Function abc()
{
    $a=15; 
    echo $a;
}

abc(); 
echo $a;

OUTPUT:

1512

Why not:

1515

Answer 1

You should use global keyword ( or sometimes $GLOBALS['a'] )

http://php.net/manual/en/language.variables.scope.php

$a=12;

function abc(){
    global $a;
    $a=15; 
    echo $a;
}

abc(); 
echo $a;

Answer 2

Because variable scope is limit to function body, you can make the variable global if you want to use the above variable like:

function abc() {
   global $a;
}