Nesting quotes, ya' need to escape 'em mate!
If you have a string wrapped in "" and would like this string to contain a literal " you will need some sort of method to say that "hey, I want this character here - but it's not the end of the string", that's where escaping comes in.
By prepending your character with a backslash \ you tell the PHP interpreter just that; this character is a part of the string, it's not used to denote the end of it.
$tag=strtolower (
str_replace ("'<<span_id=\"ctl00_cphpagecontant_label6\"><b>'","B",$tag)
);
Note: Does the string you'd like to replace start and end with '? Otherwise those characters should be removed from the string.
'<<span_id="ctl00_cphpagecontant_label6"><b>'withBreferenced as$tag?$tag's value before attempting the above? My gut-feeling is that the string is not exactly what you want to replace (<<,contantand wrapped in's all seem a little weird).<<span_id="ctl00_cphpagecontant_lalel1"><b>job_categories</b></span>><span id="ctl00_cphPageContant_FunctionalAria"><font size="2">Consultancy</font></span></<span_id="ctl00_cphpagecontant_lalel1"><b>job_categories</b></span>>