php - Which is the best - Using built in functions or writing my own? -

which 1 faster , good? writing own functions or using in built function in php eg. converting 2d array 1d array. have use

array_merge($array1) 

or writing own function using

foreach(){    } 

in general, predefined php functions faster. written in c, therefore faster, no matter how write own php functions.

if write them yourself, have more control , power on outcome. have decide whats necessary in case.