$Muestra1 = $_GET['info1']; // ex. it gets " SI"
$var1="_info1";
$contenido = substr_replace($var1, '$' . $Muestra1, 0, 0);
echo $contenido;
so my output is $SI_info1 instead of the content of the var $SI_info1

if I echo (line 4) echo $$contenido; i don't get anything
if I echo (line 4) echo "$$contenido"; i get $$SI_info1


------------- here it is again ----------

<?php
$Muestra1 = $_GET['info1'];
$var1="_info1";
$contenido = substr_replace($var1, '$' . $Muestra1, 0, 0);
echo "want to print the variable that is on \$contenido: ";
echo "$$contenido";
echo "<hr>";
echo "want to print the variable that is on \$contenido: ";
echo $SI_info1;

?>

the output is

$$SI_info1
----------------------(hr)--------
the content of the variable SI_info1