This might be a dumb question, but you do know that you can't just "cat foo.gif" in the middle of a HTML file and expect it to produce an inline image, right?

I've got two ideas:

a) unless you change it, the content-type of the results from a php program is "text/html". Try adding 
   Header("Content-type: image/gif");
to the start of your PHP file

b) the webserver hasn't got enough permissions to see the graphic? Try copying it to the same place as your php file, and typing "chmod 444 file.gif" to make it world-readable.

The following code works perfectly for me:

<?php
Header("Content-type: image/gif");
passthru("cat a.gif");
?>

I suppose there's a reason you're using "cat foo.gif" rather than readfile()?

Hi. 

I believe that I don't explain to me well, I want to send an image to the
navigator, my code in php works well if thefile that I want to show doesn't
have any space for example in the name (cat.gif), but if the name of the
image has a space (cat image.gif) the command doesn't make anything. 

Example. 
The following code converts an image using ImageMagic, the result is sent to output standar

This code don't work
<?
Header("Content-type: image/gif");
passthru("convert -scale 100x100 cat image.gif gif:-");
?>

This code work fine
<?
Header("Content-type: image/gif");
passthru("convert -scale 100x100 cat_image.gif gif:-");
?>

That's not a problem with the passthru() function- take this scenario:

------------------------------------------
test@test:/home/test$ cat test file.gif
cat: test: No such file or directory
cat: file.gif: No such file or directory
test@test:/home/test$ cat 'test file.gif'
[insert gif file here]
-----------------------------------------

So, in the function --
passthru("convert -scale 100x100 cat image.gif gif:-");

the program convert is receiving the arguments:
  0: convert
  1: -scale
  2: 100x100
  3: cat
  4: image.gif
  5: gif:-

passthru("convert -scale 100x100 'cat image.gif' gif:-");
is most likely what you want.

Your problem is nothing to do with PHP. If you ran

  cat /home/test/test file.gif

from the command prompt, it wouldn't work either. cat thinks you mean two files : "/home/test/test" and "file.gif"

You need either to escape the space, or put quotes around your filename.

Eg: 
  passthru( 'cat /home/test/test\ file.gif' );
  passthru( 'cat "/home/test/test file.gif"' );

Actually, try 2 slashes to escape the space.

$cmd = "cat foo\\ bar";

For a file named "foo bar"