Assuming you have shell access to the webserver (or at least some kind of file access) you can add a small output function at the top of the page (in the global scope):
<?php
function debug_out($msg) {
$f = fopen('/tmp/debug.log', 'a+');
if (!$f) {
trigger_error('Cannot open debug log.', E_USER_ERROR);
return;
}
fputs($f, "$msg\n");
fclose($f);
}
// rest of the PHP page below this
...
Then sprinkle the code with debug_out() calls
...
debug_out($some_variable);
...
so you can see the value of $some_variable in the output file. Add some unique identifiers to avoid confusion about which statement created the output line:
...
debug_out("\$some_variable, line 42: $some_variable");
...
Use JSON for displaying complex data structures:
...
$json = json_encode($some_variable, JSON_PARTIAL_OUTPUT_ON_ERROR);
debug_out('$some_variable, line 42: ' . ($json ? $json : json_last_error_msg()));
...
or, probably better, use print_r():
...
debug_out('$some_variable, line 42: ' . print_r($some_variable, true));
...
Note that for the latter the second parameter must be set to true, otherwise the data will be printed (i.e. shown on the page) instead of being returned and logged.
If you have shell access, /tmp is a good location for a temporary debug log file, since the directory is usually world-writable. If you don't have shell access you need to find a location where the web server can create files that you then can download for inspection.
