Most internal functions have been rewritten so they return TRUE when successful and FALSE when failing, as opposed to 0 and -1 in PHP/FI 2.0, respectively. The new behaviour allows for more logical code, like $fp = fopen("/your/file") or fail("darn!");. Because PHP/FI 2.0 had no clear rules for what functions should return when they failed, most such scripts will probably have to be checked manually after using the 2.0 to 3.0 convertor.
Example#1 Migration from 2.0: return values, old code
$fp = fopen($file, "r");
if ($fp == -1);
echo("Could not open $file for reading<br />\n");
endif;
Example#2 Migration from 2.0: return values, new code
$fp = @fopen($file, "r") or print("Could not open $file for reading<br />\n");