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<?php $classmap1 = array(); for ($i=1; $i<=10; $i++) { $classmap1['a'.$i] = $i+1; } $classmap2 = array(); for ($i=1; $i<=10; $i++) { $classmap2['b'.$i] = $i+1; } $startTimeMerge = microtime(true); $map = array_merge($classmap1, $classmap2); $endTimeMerge = microtime(true); $startTimeForeach = microtime(true); foreach ($classmap2 as $k => $v) { $classmap1[$k] = $v; } $endTimeForeach = microtime(true); echo "Merging took " . (($endTimeMerge - $startTimeMerge)*1000) ." microseconds<br />\n"; echo "Foreach took " . (($endTimeForeach - $startTimeForeach)*1000) ." seconds<br />\n";

preferences:
32.23 ms | 402 KiB | 5 Q