PHP: Call to undefined function gzdecode()

I am getting a strange error message with the following piece of PHP code (I am not a PHP expert):

if ( $file_loc != NULL ) {
    if ( file_exists($file_loc) ) {
        printf(" file exists");
        $handle = fopen($file_loc, "rb");
        $contents = fread($handle, filesize($file_loc));
        fclose($handle);
        $result = gzdecode($contents);
    }
}

I am basically trying to load text content from a gzipped file. I get the following error:

Fatal error: Call to undefined function gzdecode() in ...\sites\MyScripts\fw2.php on line 80

Yet, when I take a look at documentation, it does not seem like I would need to include an extra library, or am I being wrong? How can I solve this issue?

UPDATE

Following another question to check whether this library is installed on my PC, the answer is yes, it is.

From PHP info:

So this is getting more and more confusing...

UPDATE II

I have tried:

<?php

echo phpversion().", ";

if (function_exists("gzdecode")) {
  echo "gzdecode OK, ";
} else {
  echo "gzdecode no OK, ";
}

if (extension_loaded('zlib')) {
  echo "zlib extension loaded ";
} else {
  echo "zlib extension not loaded ";
}

?>

and I get:

5.2.17, gzdecode no OK, zlib extension loaded