html - PHP NULL vs 'NULL' -

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this question came me spending 2 hours of trial , error , finding bug is, , need clarify why working way.

the code below prints out dropdown box default string being '--select status--' value of null. 

print("<label>overall status overwrite:</label>        <select name='case_ov_status' class='case_ov_status'>        <option selected='selected' value=null>--select status--</option>");

on submit, call function determine whether dropdown box @ default value, if not update sql database. however, i'm confused. if statement below still run despite submitting value of null. found out 2 hours later using if($case_ov_status != 'null') instead of if($case_ov_status != null) solved problem. 

if($case_ov_status != null){ //still ran despite != null.                 mysql_query("start transaction", $connection);                 $sql = "update cases set status=".$case_ov_status." patientid='".$patientid."' , caseid='".mysql_real_escape_string($case)."'";                 $resultnew = mysql_query($sql, $connection);

this may sound basic, can explain how works? still learning.. in advance!

please read http://php.net/manual/en/language.types.null.php

null special value represent unallocated objects in php. "null" string literal containing word null , not same. in php may see type juggling equality. example, "" == null true.

you cannot send php null value on http since string in http. form though specify null value, send "null" server.


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