PHP Keywords & Identifiers
$identifier = value; // identifier: letter or _ first, then letters, digits, _
function identifier() { }
class Identifier { }Keywords
Keywords are words PHP has already claimed a specific meaning for — if, else, function, class, and dozens more.
Because the interpreter uses these words to control program flow, you can't reuse one as a variable or function name; doing so causes a parse error.
Example: Keywords
<?php
if (true) {
echo "if, else, function, class are reserved keywords";
}
// $if = 5; would cause a parse error -- keywords can't be variable names
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Identifiers
An identifier is any name *you* choose — for a variable, a function, or a class — so PHP knows what to call the thing you've created.
A clear identifier like $totalPrice documents intent on its own, saving a reader from having to guess what a vaguer name like $x is supposed to hold.
Example: Identifiers
<?php
$totalPrice = 49.99;
echo $totalPrice;
// A clear name documents intent better than $x would
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Naming Rules
PHP requires identifiers to start with a letter or an underscore, and only letters, digits, and underscores after that — no spaces, hyphens, or special symbols.
This is a hard syntax rule, not a style preference: $user-name will fail to parse as a single identifier.
Example: Naming Rules
<?php
$user_name = "Alice";
echo $user_name;
// $user-name would fail to parse -- hyphens aren't allowed in identifiers
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Case Sensitivity in Names
Variable names are case-sensitive in PHP, so $page, $Page, and $PAGE are three entirely separate variables that don't share a value.
This trips up beginners more than any other casing rule in the language, since function and keyword names *don't* behave this way.
Example: Case Sensitivity in Names
<?php
// Declare `$page`, set to "home"
$page = "home";
// Declare `$Page`, set to "about"
$Page = "about";
// Declare `$PAGE`, set to "contact"
$PAGE = "contact";
// Print "$page, $Page, $PAGE are three separate variables" to the output
echo "$page, $Page, $PAGE are three separate variables";
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Best Practices for Names
Favor names that describe what a value represents — $userEmail over $e — especially for anything you'll reference more than once.
The extra characters cost nothing at runtime, and they save real time for whoever (including future you) has to read the code later.
Example: Best Practices for Names
<?php
$userEmail = "[email protected]";
echo $userEmail;
// Preferred over a vague name like $e
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- Starting a variable name with a digit, such as
$1st_place, which is a parse error because names must begin with a letter or underscore. - Using a reserved keyword like
classorfunctionas a constant or function name, which produces a syntax error. - Using
$user-namewith a hyphen, which PHP reads as$user - name(subtraction), not one variable name.
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