eval function - I don't know what I am doing wrong...

["gregaizi" <gregaizi@xxxxxxxxx>]

All manual suggest about the same code as examples:
<code>
<?php
$name = 'Mary';
$name2 = 'Ana';
$a = 'My friends are called $name and $name2';
print $a . "<br>";
eval("\$a = \"$a\";");
print $a . "<br>";
?>

</code> 
and it works.

But it does not work in my code
<code>
$s=file_get_contents($_GET['p']); //PAGE
if (!eval("\$str = \"$s\" ;")) {fwrite($f,"error evaluating!\n"); $errorflag=true; } 
</code>
the content of the $s is a php-program ECHOing some data...

Or how I can call php for a program, catch and use all of its output in the calling php-program?

Thanks in advance