Strip equal sign when inserting formulas
Created by: stoppeye
The equal sign should be stripped when inserting a formula into <f> tags (see Reference).
While Excel will still handle it correctly, it can cause errors in other applications (e.g. Google Sheets).
Adding ltrim on the value can fix this:
} elseif (is_string($value) && $value{0}=='='){
$file->write('<c r="'.$cell_name.'" s="'.$cell_style_idx.'" t="s"><f>'.self::xmlspecialchars(ltrim($value, '=')).'</f></c>');