php loop with letters without using an array of letters - php

Question

I have an php code that writes an excel file with a variable number of columns. Each 4 rows, on the 5th I want to put the total of the four rows above, per column.
My issue is on how to write the formula in terms of columns.
My code is this one:
$col_index=20;
$i=20;
for($anno = $annoMin; $anno<=$annoMax; $anno++){
for($mese = 1; $mese <= 12; $mese++){
$string = "=$i$v+$i$q-$i$z-$i$t";
$ews->setCellValueByColumnAndRow($col_index,$k,$string);
$col_index=$col_index+1;
$i=$i+1;
}
}
In $string I need to put the column letter instead of $i. $v,$q,$z and $t are the reference to the rows and are ok. In other words $string should be evaluated to:
$string = "=U5+U3-U2-U4";
during the first for loop,
$string = "=V5+V3-V2-V4";
in the second and so on.
I know I can build an array of columns letter and use that $i reference to get my goal but I'm sure there is a better approach. I am using php 5.5 btw
The other option is to write the formula with the R1C1 notation but I'm pretty sure I cannot have the standard and the R1C1 notation in an excel sheet at the same time

Answer 1

You can increment a string variable. I think this is the most efficient way to do what you need (if I understood it right).
$ch = "a";
++$ch;
echo $ch; //prints "b"
Knowing this you can build a loop to update the value as you need.
This works very good for Excel because:
$ch = "z";
++$ch;
echo $ch; //prints aa

Answer 2

I'm just concentrating on the column letter as that seems to be what you're asking. Use the ASCII code of the letter and increment it, then convert it to the character:
$col_index = 20;
$i = ord('U'); // 85
for($anno = $annoMin; $anno<=$annoMax; $anno++){
for($mese = 1; $mese <= 12; $mese++){
$c = chr($i);
$string = "=$c$v+$c$q-$c$z-$c$t"; // $i will be U then V etc...
$ews->setCellValueByColumnAndRow($col_index,$k,$string);
$col_index = $col_index+1;
$i++;
}
}
If you need to keep $i starting at 20 and incrementing then just add 65:
$col_index = 20;
$i = 20;
for($anno = $annoMin; $anno<=$annoMax; $anno++){
for($mese = 1; $mese <= 12; $mese++){
$c = chr($i + 65);
$string = "=$c$v+$c$q-$c$z-$c$t"; // $i will be U then V etc...
$ews->setCellValueByColumnAndRow($col_index,$k,$string);
$col_index = $col_index+1;
$i++;
}
}