PHP Debugging Techniques
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var_dump($variable);
print_r($variable);
debug_backtrace();
if (DEBUG_MODE) {
var_dump($variable);
}Simple Print Debugging
var_dump() and print_r() are the fastest way to inspect a variable's actual contents -- var_dump() additionally reveals data types and array/object structure, which matters when a bug turns out to be a type mismatch rather than a wrong value.
Example: Simple Print Debugging
<?php
// Declare `$data` as an array: `["id" => 1, "active" => true]`
$data = ["id" => 1, "active" => true];
// Print a detailed dump (with types) of `$data`
var_dump($data);
// Print a human-readable dump of `$data`
print_r($data);
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The Backtrace Utilities
debug_backtrace() returns the full chain of function calls that led to the current line, which is invaluable when you land inside a deeply nested call and need to understand how execution got there.
Example: The Backtrace Utilities
<?php
// Define the function `a` with no parameters
function a() { b(); }
// Define the function `b` with no parameters
function b() { print_r(debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS)); }
// Call `a()`
a();
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Conditional Debugging
Wrapping debug output in a check against a constant like DEBUG_MODE lets you leave diagnostic code in place without it ever running (or leaking information) in production.
Example: Conditional Debugging
<?php
// Call `define('DEBUG_MODE', true)`
define('DEBUG_MODE', true);
// Check whether `DEBUG_MODE`
if (DEBUG_MODE) {
// Print "Debug: variable value is 42" to the output
echo "Debug: variable value is 42";
}
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Tracking Memory and Time
memory_get_usage() and microtime(true) let you measure exactly how much memory a script consumes and how long a section takes to run, turning vague 'this feels slow' complaints into concrete numbers you can optimize against.
Example: Tracking Memory and Time
<?php
// Declare `$start`, set to `microtime(true)`
$start = microtime(true);
// Declare `$mem`, set to `memory_get_usage()`
$mem = memory_get_usage();
for ($i = 0; $i < 10000; $i++) {}
// Print "Time: " . round(microtime(true) - $start, 4) . "s\n" to the output
echo "Time: " . round(microtime(true) - $start, 4) . "s\n";
// Print "Memory: " . (memory_get_usage() - $mem) . " bytes" to the output
echo "Memory: " . (memory_get_usage() - $mem) . " bytes";
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Safe Variable Inspection
var_export() produces a string of valid PHP code representing a variable's value, which is handy for generating cache files or config arrays, not just for eyeballing a value during debugging.
Example: Safe Variable Inspection
<?php
// Declare `$config` as an array: `["theme" => "dark", "retries" => 3]`
$config = ["theme" => "dark", "retries" => 3];
// Print `var_export($config, true)` to the output
echo var_export($config, true);
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- Leaving
var_dump()orprint_r()calls in code that goes to production, which shows internal data to visitors. - Debugging output before
header()calls, which triggers a headers already sent warning. - Forgetting
<pre>aroundprint_routput in a browser, so the layout is unreadable.
- Error handling begins with understanding PHP's errors, and
try-catchhandles exceptions. - Custom exceptions create specific error types.
- Error logging and debugging techniques help you find problems.
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