Cast "1+2" string to integer value of 3 in PHP

Question

Is there a way to cast "1+2" string into integer value of 3?

$a = "1+2";
echo (int)$a;            // 1
echo ( $a * 1 );         // 1
eval("\$a = \"$a\";");
echo $a;                 // "1+2"

I tried googling with no success.

Answer 1

$a = "return " . $a . ";";
echo eval($a);