I'm using imagettftext in php to generate images. Is there a true type font that supports all utf-8 characters?
As Brian Neal stated in his comments, UTF-8 is just one of several encoding standards that can represent every character in the Unicode character set, which currently contains more than 100000 entries.
So you are actually asking for a true type font that supports all 100k+ unicode characters currently in use on this planet (and it's a moving target, as the set gets expanded and adjusted constantly).
So I guess the literal answer is no, and you should probably check your preconditions (what character subsets are likely encountered in your use case), and search for a fitting multi-purpose font.
However, there are attempts to provide fonts that cover large amounts of the unicode space - search for 'Pan-Unicode Fonts' to get an overview.
☕ (U+2615 - 'Hot Beverage' ;)