RTFM:

http://www.php.net/manual/en/types.comparisons.php

(never ever compare different types like that)

Documentation needs to be more explicit about php being strongly typed.

http://www.php.net/manual/en/language.types.type-juggling.php

or

http://www.php.net/manual/en/language.types.string.php#language.types.string.conversion

"NANC" is converted to 0 for comparison. It works for me as expected. What is printed by the code <?php echo intval("NANC"); ?> on your system?

It also print 0 as expected.

if( (int) "NANC" < 0)

also evaluates as expected.

Vrana: your comment makes no sense. If "NANC" is converted to 0, then 0 < 0 would be FALSE, not true.

Compare these expressions:
"NANC" < 0    result TRUE
"NANC" < "0"  result FALSE
0 < 0         result FALSE

So it seems it is neither converting "NANC" to an integer 0, nor 0 to a string "0". What actually is going on? The documentation should clarify this.

Who wants to evaluate "NANC" < 0??? It _absolutely_ makes no sense. If someone asks you if A+B > 4 is, you wouldn't know what to calculate...

FYI I've tried ivo's script. Under PHP 4.3.3 it produces "should happen...but doesnt" and under 4.3.0 it's "should not happen...but does". 

> If someone asks you if A+B > 4 is, you wouldn't know what to calculate...

I sincerely hope this would hold:

http://www.php.net/manual/en/language.operators.php#language.operators.precedence

....otherwise theres bigger problems.

If a statement such as 

"some arbitrary string" < 0

is not to be trusted then the documentation should be updated to reflect this. Currently it explains strong comparisons with '===', weak with '==' but it does not mention < or >, ony obliquely and it obviously does not.

(http://www.php.net/manual/en/types.comparisons.php)

p.s. I am working with a site that manipulates strings everywhere. most functions return < 0 on error or the processed string if all goes well. just finished updting it to return ints always & use references for strings.

still think it's an outright bug or at least a documentation problem due to all the 'weak' type comparison references & examples everywhere.

As said by sniper: never ever compare different types like that!!!

In case $var is an int, you can do a $var < 0. In case $var is  a string, PHP don't know what to do (try this with Java or C++!).

Let your called functions return FALSE on error and everything works fine. Or as an alternative, try something like 

$var=somefunction();
if(is_integer($var)&&$var<0)
{
   // Here comes the error-Processing
   print("Error\n");
}
else
{
   // Do something with $var
   print $var;
}