If $a = 5, $b = 'a', what is the value of $$b?

Explain this interview question to me:

Q: If the variable $a 开发者_开发技巧is equal to 5 and variable $b is equal to character a, what’s the value of $$b?

A: 5, it’s a reference to existing variable.

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That's a variable variable. PHP will look up the variable with the name stored in the string $b. So if $b == 'a' then $$b == $a.

It's a lot like pointers in C, except they use variable name strings instead of memory addresses to point to each other. And you can dereference as many times as you want:

$a = 5;
foreach (range('b', 'z') as $L) {
  $$L = chr(ord($L) - 1);
}
echo $$$$$$$$$$$$$$$$$$$$$$$$$$z;

Output:

5

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-95 is the answer as if u will echo $b u will get output as "a" and if u echo $a u will get out but as "5"

hence in this sense when u $(echo $b) which same as $(a) hence u will get it as "5-100" which is "-95"

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  $$b - 100
= $a - 100 // substituting $b=a
= 5 - 100
= -95

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I don't know if the '?' is erroneous in the statement '$$b? - 100' but I don't think that will compile.

However:

$a = 5
$b = 'a';
$c = $$b - 100;

$c will equal -95, because $$b is a variable variable reference and given that $a = 5 it resolves to $a (5) - 100, or -95.

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the answer is -95

$a - 100

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The following is a good reference on PHP variables

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