PHP a lightweight language?

I'm interested in the proposition made recently on this list that PHP is somehow a 'lightweight' language that some people don't take seriously. I am relatively new to Linux and open source programming and only really know PHP and some shell scripting, though I am now starting to explore Python. I have had a quick overview of several languages and it seems to me that they all deal with the same basic components and logic: variables statements flow control functions classes etc etc etc So I'm wondering, apart from the fact that PHP is web centric, what features does the language lack that prevent some from taking it seriously? It appears to me to use the same basic elements and logic as any other programming language. Maybe it doesn't have tedious requirements for memory management like C, but so what? Neither does Python. Any comments appreciated. Mick