PSR suggests, method names MUST be declared in camelCase and class names MUST be declared in StudlyCaps.
In PSR-12 there is an explanation of what they meant by StudlyCaps:
The term ‘StudlyCaps’ in PSR-1 MUST be interpreted as PascalCase where the first letter of each word is capitalized including the very first letter.
source: https://www.php-fig.org/psr/psr-12/#21-basic-coding-standard
P.S.
For clarity, there are two alternatives for camel case - upper camel case (initial uppercase letter, also known as Pascal case) and lower camel case (initial lowercase letter, also known as Dromedary case).
Some people and organizations, notably Microsoft (and seems the authors of PSR-1 too), use the term camel case only for lower camel case. Pascal case means only upper camel case.