Description:
------------
When using function function_exists() to test for presence of a function, the result is sometimes wrong.

Consider the following:
echo "a:", function_exists(imagecreate),"<br>";
echo "b:", function_exists(imagecreatetruecolor),"<br>";
echo "c:", function_exists(imagecreatetruecolorrrr),"<br>";
echo phpinfo(8);

This produces the output:
a: 1
b: 1
c:
<snip>

gd
GD Support	enabled
GD Version	1.6.2 or higher
FreeType Support	enabled
FreeType Linkage	with freetype
JPG Support	enabled
PNG Support	enabled
WBMP Support	enabled

When I call imagecreatetruecolor(), I get:
Warning: imagecreatetruecolor(): requires GD 2.0 or later in /home/gregt/public_html/gd/gdtest.php on line 8

Obviously I don't have a proper implementation of the imagecreatetruecolor function, but there is still code there to throw an error, so function_exists blindly assumes the function is present.

Reproduce code:
---------------
<?
echo "a:", function_exists(imagecreate),"<br>";
echo "b:", function_exists(imagecreatetruecolor),"<br>";
echo "c:", function_exists(imagecreatetruecolorrrr),"<br>";
echo phpinfo(8);

?>

Expected result:
----------------
a: 1
b:
c:



Actual result:
--------------
a: 1
b: 1
c: