use strict;
use warnings;
use 5.010;


my $fname = 'somefile.php';

#Slurp whole file:

my $file;
{
    local $/ = undef;
    $file = <DATA>;
}

my %files_for;

while($file =~ m{  
                   <[?]php
                   \s* 
                   (.*?)
                   \s* 
                   [?]>  
    
                }xmsg) 
{
    my $php_code = $1; 
    push @{ $files_for{$php_code} }, $fname;
}

use Data::Dumper;
say Dumper(\%files_for);
    
__END__
<div>hello</div>
<div><?php echo 'world'; ?>
<div><?php echo 'hello';?>
<div><?php echo 'world';?>


--output:--
$VAR1 = {
          'echo \'world\';' => [
                                 'somefile.php',
                                 'somefile.php'
                               ],
          'echo \'hello\';' => [
                                 'somefile.php'
                               ]
        };