if statement with multiple condition in php - php

Question

I have if statement with multiple condition,
what is the differece between this two conditon:
1.
if($province=="AB" || "NT" || "NU" || "YT")
{
$GST=5;
}
else if($province=="BC" || "MB")
{
$GST=5;
$PST=7;
}
else if($province=="NB" || "NF" || "ON")
{
$HST=13;
}
and second is:
2.
if($province=="AB" || $province=="NT" || $province=="NU" || $province=="YT")
{
$GST=5;
}
else if($province=="BC" || $province=="MB")
{
$GST=5;
$PST=7;
}
else if($province=="NB" || $province=="NF" || $province=="ON")
{
$HST=13;
}

Answer 1

The difference between the two is that the first one won't work as expected, and the second is technically correct.
The code:
if($province=="AB" || "NT" || "NU" || "YT")
will always evaluate to true and execute the code in that conditional block.
The reason is because you are only checking if $province == "AB" and then you are checking if "NT" == true which will evaluate to true.
To check province against all of those values (AB, NT, NU, YT) you need to explicitly check $province against each value, not just the first, which is what you are correctly doing in the second example.

Answer 2

I add to drew010 answer that you can do this too (wich is simpler) :
if(in_array($province,array("AB","NT","NU","YT"))
{
$GST=5;
}
else if(in_array($province,array("BC","MB"))
{
$GST=5;
$PST=7;
}
else if(in_array($province,array("NB","NF","ON"))
{
$HST=13;
}

Answer 3

The first one will always evaluate to true. In the second, third, and forth OR clauses in the first example you are basically asking PHP to convert the strings to Boolean values, and non-empty strings will always evaluate as true.
Just for fun, my favorite way to handle conditionals like this is with a switch statement. It's a lot easier for me to read, but it's really a personal preference thing.
switch ( $province ) {
case 'AB' :
case 'NT' :
case 'NU' :
case 'YT' :
$GST = 5;
break;
case 'BC' :
case 'MB' :
$GST = 5;
$PST = 7;
break;
case 'NB' :
case 'NF' :
case 'ON' :
$HST = 13;
break;
}