it seems that PHP4RC1 doesn't evaluate logical statements in the same way as PHP3 did . this causes some errors which (at first glance) looked to me as MySQL driver errors where a valid SQL statement, when implemented in a code similar to this : 
if ($result = mysql_query($query) && $row = mysql_fetch_array($result)) { 
would return a mysql warning : "Supplied argument is not a valid MySQL result ersource" on the mysql_fetch_array command. 
I think this happens because PHP4 doesn't evaluate the mysql_query part before the mysql_fetch_array part, which results in the fact that the $result variable isn't set in time for the mysql_fetch_array command. 
this behavior could be well demonstrated by the two following pieces of code : 
1) 
$t = 1; 
if ( $t = $t + 1 && $w = $t + 1 ) { 
echo "right ! t: $t w: $w"; 
} else { 
echo "wrong! t: $t w : $w"; 
} 

2) 
$t = 1; 
if ( ($t = $t + 1) && $w = $t + 1 ) { 
echo "right ! t: $t w: $w"; 
} else { 
echo "wrong! t: $t w : $w"; 
} 

which although they look semanticly the same give different results.