that is quiet common mistake
is that because the people in south china can't curl their
tongue?
thus most singaporean pronociate "th"
[θ] as "ph" [f]
and for mandrain , looks like no different about :
shi 十and si 四
quite true
Singaporeans pronounce written "th" [θ] initially as "t"
and finally as "f"
therefore "three" becomes "tree" and
"teeth"
become "teef"
the reason behind that is [θ] doesn't exist in the most people's
mother tongues of Singaporeans (all Southern Chinese dialects, all
Malay dialects, all Indian languages , only Arabic has [θ]
sound) that's why, therefore the substitution of similar phonemes
in place of it
similarly another phenomenon is the pronunciation of final and
sub-final "l" or "ll" as "u"/"w"
eg.
kill --> kiu
pill --> piu
milk --> miuk
heal --> hiu
until -->
untiu
file --> fau
pail --> pehw
pale --> pehw
mail --> mehw
fail --> fehw
cold --> cou
hold --> hou
fold --> fou
hole --> hou
shield -->
shiu
pull --> puu
full --> fuu
tool --> tuu
but usually I hear Indians can pronounce the actual final "l"
due to their mother tongue having final "l" too. but Chinese
usually pronounce as above.