Case-sensitive are not applicable to all PHP keywords (i.e if, else, while, echo, etc.), classes, functions, and user-defined functions.
<?php
ECHO "Hello Lazacode!<br>";
echo "Hello Lazacode!<br>";
EcHo "Hello Lazacode!<br>";
?>
However; all variable names are case-sensitive. The following example of variables treated as 3 different variables.
<?php
$color = "brown";
$COLOR = "black";
$coLOR = "white";
echo "My hair color is " . $color . "<br>";
echo "My motorbike color is " . $COLOR . "<br>";
echo "My house color is " . $coLOR . "<br>";
?>