Perl vs. PHP : variable scoping
I’ve mentioned quite a few times that I am a big fan of Perl programming languge. However, most of my programming time these days is spent in PHP. The languages are often similar, with PHP having its roots in Perl, and Perl being such a influence in the world of programming languages. This similarity is often very helpful. However there are a few difference, some of which are obvious and others are not.
One such difference that I came up recently (in someone else’s code though), was about variable scoping. Consider an example in Perl:
#!/usr/bin/perl -w
use strict;
my @values = qw(foo bar hello world);
foreach my $value (@values) {
print "Inside loop value = $value\n";
}
print "Outside loop value = $value\n";
The above script will generate a compilation error due to undefined variable $value. The one outside the loop.
A very similar code in PHP though:
#!/usr/bin/php
<?php
$values = array('foo','bar','hello','world');
foreach ($values as $value) {
print "Inside loop value = $value\n";
}
print "Outside loop value = $value\n";
?>
Will output the following:
Inside loop value = foo Inside loop value = bar Inside loop value = hello Inside loop value = world Outside loop value = world
In Perl, variable $value is scoped inside the loop. Once the execution is out of the loop, there is no such thing as $value anymore, hence the compilation error (due to the use of strict and warnings). In PHP, $value is in global scope, so the last value “world” is carried further down the road. In case you reuse variable names in different places of your program, counting on scope to be different, you might get some really interesting and totally unexpected results. And they won’t be too easy to track down too. Be warned.