It seems to affect any E notation string being cast to float.

var_dump(exp(10), (string)exp(1), exp(1) == (string)exp(1), exp(1) - (string)exp(1));

Result:

float(2.718281828459)
float(2.718281828459)
bool(false)
float(4.5297099404706E-14)

Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is, read this:
http://www.floating-point-gui.de/

Thank you for your interest in PHP.

The exact values depend on your precision and serialize_precision settings.

$ php -i | grep precision
precision => 14 => 14
serialize_precision => 17 => 17

$ php -r 'var_dump(...);'
float(2.6881171418161E+43)
string(19) "2.6881171418161E+43"
bool(false)
float(3.5652673131419E+29)

$ php -d precision=20 -d serialize_precision=25 -r 'var_dump(...);'
float(2.6881171418161356094E+43)
string(25) "2.6881171418161356094E+43"
bool(true)
float(0)

To be clear it's the precision that matters here, but if that's a concern then you should consider the impact of serialize_precision too.