Reasons PHP is not great

Picking up proper coding practices that promote maintainability, 
security, simplicity, resiliency, etc. in any language requires time and experience

Incomplete documentation

User-provided comments in the online documentation are frequently inaccurate. Arguably, this actually hinders one's ability to learn how to write proper PHP

encourages the mixing of business logic, database calls and presentation code.

inconsistent naming of functions

arguments to functions are inconsistent

case sensitivity--doesn't matter for functions, but does for variables

non-backward compatible changes to the language

character-set support

isn't thread safe

session handling is single-threaded

support for large integers varies between 32 and 64 bit systems (and unicode)

sacrifice clarity for brevity

coding standards

duplicate code

development patterns

try to do it all yourself

sharing

bad recursion

no namespaces

non-standard date format characters

magic quotes

slow

not filtering user input